These answers pretty well cover several of your options, but I'd like chip in a few others.
Targeting
As others have said, be aware of how the spell is cast. You must be within 30' of your initial target, must have line of sight on an initial mind-reading target, then must use an Action to switch it to a new target. OR you may scan for thoughts within 30' of you and use an Action to pick one of those thought sets to chose from
So, you have three options here.
1: You cast the spell, involving speech, gesticulation, and a copper coin within line of sight of your target. This is super suspicious. And will probably end in combat or summoned guards. And, however it pans out...the spell will have expired by the time things calm down.
2: You cast the spell outside the line of sight of the target, then move to within line of sight and shift the spell, this burns a round of time (I'll get to that in a second.) Assuming it only takes you 1 round to get into line of sight of the target. And, bear in mind, someone else might see you casting and freak out. I'd be calling for stealth checks here if they were in a populated area.
3: You cast the spell outside the line of sight of the target, then attempt to hone in on the thoughts of your target without moving into line of sight. The catch here is, you detect every intelligent creature within 30' and have to pick which one you want to focus on, consuming an Action. This includes your allies and any bystanders, and the spell does not give you any information about the intelligent creatures besides 'they are within range of the spell.' You don't even know which direction they are in. If this is how they are doing it, determine how many people are within range of the casting, and roll the dice to see if they picked the right person.
Time
The spell lasts for 60 seconds. 10 rounds. Most conversations last significantly longer than that. And unless they cast the spell directly on the target (requiring line of sight) then they burn 6 seconds to take a round and Action to switch the spell to their desired target. So, more likely, they can Detect Surface Thoughts for 54 seconds.
So, have your NPCs engage in small talk...like actual people would. If the players try to push them straight to the topic, feed the person reading their mind thoughts about how rude they think the players talking to them are...and allow that to run a risk of the NPC refusing to talk to them further.
In social interaction, 60 seconds is not a long time. Particularly if the players cast the spell before trying to start the conversation. Have the NPC putter around their shop for a bit before coming to talk to them...have them be already talking to someone else when they arrive...have an interruption occur (such as someone else entering the shop). Anything that eats up time. They are taking a gamble by casting Detect Thoughts that the relevant topic will be discussed within 60 seconds. This isn't an interrogation where you just start demanding answers...and if they treat it like one, I'd expect no one to want to talk to the players next time they are in town.
Obviously, don't abuse this...let Detect Thoughts work sometimes. Otherwise you'll just frustrate them. But it shouldn't be the all-solving-hammer they are currently using it as.
Hard Counters
If you have a villain, you want to block this, especially if they are going to deceive the players directly. For this, you need hard counters that absolutely block mind reading. You have a few options...
The obvious option here is Mind Blank. You are immune to mind reading. Period. They try to read your mind, and get nothing.
Next is the 17th level Mastermind-Archetype Rogue's ability, Soul of Deceit. They can block attempted mind reading with ease, or even roll Bluff to present false thoughts to attempts to do so.
Creative use of Modify Memory can bypass this. The villain could temporarily alter their memory (leaving a written note to tell them to undo the spell later) to believe they are telling the truth. Thus, 'Detect Thoughts' would accurately pick up on false information.
Mordenkainen's Private Sanctum shuts down Detect Thoughts entirely...one of the options is that no creature inside the Sanctum can be targeted by a Divination spell. You could even play this off as the other person casting or entering an area under the effect of the spell to 'make sure their conversation remains private.' The RAW allows for permanent versions of these...so a wealthy non-magic user could very well have a permanent Sanctum set up for private conversations.
Nondetection is probably the lowest level method of a hard-block. It makes the target immune to Divination spells (and, again, Detect Thoughts is a Divination Spell).
And, of course, Detect Magic would go off if a player using Detect Thoughts entered the caster's range. It wouldn't tell them specifically what you were up to, but they would know you were using a Divination spell in their presence.
Non-Spell Countermeasures
Misinformed NPC. Have the character providing the information be entirely honest and truthful...but the information they give was provided by a deceptive source, so the NPC in question is honest...but wrong. The players may be able to use DT to be immune to manipulation, but the NPCs they talk to certainly aren't.
Pathological Liar. A character like this would probably think about several different 'answers' to their question with no distinction as to which one was true. You know they probably aren't telling you the truth...but good luck figuring out which of their thoughts was the truth.
Superfluous information. People's brains hop around all the time. Don't just give your players the relevant information..dump that NPC's whole stream of consciousness on them. Thinking about what's for dinner, wondering if these adventurers will shut up and buy something, grumping about that spider web they just spotted over your shoulder (I just swept the corners, dangit). Sure...they might get the info they are looking for, but they'll have to dig through a bunch of extra crap to find it.
Hopefully this helps out a bit
Squeezing
what if you arrange your panels so there's a section only 2" wide? Is the creature squeezed between the walls?
First, the space a creature takes up is not the same as the size the creature controls.
A creature's space is the area in feet that it effectively
controls in combat, not an expression of its physical
dimensions. A typical Medium creature isn't 5 feet
wide, for example, but it does control a space that
wide. If a Medium hobgoblin stands in a 5-foot-wide
doorway, other creatures can't get through unless the
hobgoblin lets them. (PHB 192)
The rules on squeezing are pretty clear for most cases:
A creature can squeeze through a space that is large
enough for a creature one size smaller than il. Thus, a
Large creature can squeeze through a passage that's
only 5 feet wide. While squeezing through a space,
a creature must spend 1 extra foot for every foot it
moves there, and it has disadvantage on attack rolls
and Dexterity saving throws. Attack rolls against the
creature have advantage while it's in the smaller space. (PHB 192)
So, a medium creature can squeeze into a space that is Small, which according to DMG page 6 is a 2.5x2.5 ft square it controls.
Squishing
2 feet is less than 2.5 feet so what happens? The rules simply don't say, and a ruling is necessary. So here are some options the DM might rule.
The Creature Takes Bludgeoning Damage
The DM could rule that like in real life when getting stuck in a space that's too small, the creature gets squished into the space, all the things of being squeezed applied, and take some amount of bludgeoning damage. They may also say the space so cramped he's not just losing a foot of movement, but maybe he's in difficult terrain or even restrained.
The Creature is Pushed The Other Way
The DM could rule there isn't enough room and spell isn't a damage spell, and the spell simply shunts them the other way instead.
The Creature is Squeezed
The DM could rule that sizes are an approximation and abstraction. All medium creatures fit inside of a 5' square, but maybe the elf has a slight enough frame can squeeze not only into a 2.5' square but even into a 2' square. The DM could continue that logic to even smaller spaces if they felt appropriate.
Best Answer
Fight magic with magic
There are many spells available to spellcasters to either stop or suppress the spells in this combo. These options require the BBEG to use up spell slots (potentially high level spell slots), and many of them will require the BBEG to be exposed to the combo for at least one round.
If the BBEG is not a spellcaster and does not have magic items which can cast some of these spells on their behalf, their options are sorely limited.
Counterspell
If the players are within 60 feet of the BBEG when they enact this plan, the BBEG will want to cast counterspell. Since the player characters are readying an action to cast a spell, it should be obvious to any BBEG worth their salt that they are about to enact some nasty combo and that they should stop that.
However, the BBEG only gets one reaction, so they can only counter one spell. There is the choice of countering wall of force or sickening radiance. They can only stop one spell. If the party did not coordinate their actions, they may have allowed the BBEG a turn between the two casters such that the BBEG could counter both spells. Thus counterspell rewards the players for good teamwork while preventing them from trivialising the encounter.
Note that if Readying a spell is involved, counterspell must be used when the spell is Readied, because that is when casting takes place.
Dispel Magic
If the BBEG is hit by this combo, they can cast dispel magic on sickening radiance. They are still stuck inside a wall of force, which is not a good place to be, and they may have taken damage from one round of sickening radiance if they failed their Constitution save. The BBEG's escape will be sufficiently delayed that the party might gather into a more advantageous position.
Disintegrate
If the BBEG is high enough level to have disintegrate, then they can use it to get rid of the wall of force. They can then walk out of the sickening radiance, although they'll still have faced at least one round of sickening radiance (depending on their speed). Forcing the BBEG to use their disintegrate on a wall of force and not a player character is a substantial advantage.
Antimagic Field
The nuclear option for high-level spellcasters fighting against other spellcasters. This will allow the BBEG to completely ignore both sickening radiance and wall of force. However, it also suppresses any buff spells the BBEG may have had active and prevents them from casting other spells, although they can always drop concentration on antimagic field once they are clear of the combo. This costs an eight level spell slot, though, so is a very costly counter to the combo.
Globe of Invulnerability
This spell has a similar effect to antimagic field, in that it completely negates wall of force and sickening radiance, but it uses a lower level spell slot (although at 6th level its still significant) and doesn't stop the BBEG from casting spells. But the casters can also keep casting spells at the BBEG by moving to within 10 feet of the BBEG.
Etherealness
Wall of force prevents escape via the Ethereal Plane, but side-stepping to the Ethereal Plane allows the BBEG to ignore sickening radiance. Etherealness's 8 hour duration allows the BBEG to wait out the spells of the combo and then move into a more advantageous position (or possibly flee). This requires the BBEG to spend a 7th level spell slot.
Harm the casters, break their concentration
The BBEG might have some way to harm the characters without attacking them directly. BBEGs regularly have minions (and if your BBEG doesn't have minions, why not?). The BBEG would order their minions to focus fire on the two casters, attempting to break their concentration.
However, you specify solo BBEG. If adding minions is not an option, you can instead consider adding lair actions. Lair actions will function as long as the BBEG is alive and can be used to affect the party even if the BBEG can't reach them personally.
Until such time that concentration is broken, though, the BBEG is trapped, unable to fight directly, and making saves against sickening radiance. Depending on circumstances, it might be many rounds before the BBEG can escape. This rewards the party's ingenuity.
Teleport out of there
Wall of force does not block teleportation (unlike its higher level counterpart forcecage). If the BBEG has spells such as dimension door or misty step, they can escape the wall of force and continue the fight. (Note that misty step has a range of Self, meaning it is unaffected by wall of force providing total cover.) Misty step is a rather cheap counter, being only a second level spell and a bonus action, but it is also a commonly available one. Dimension door, being only a fourth level slot, also causes the players to have spent more resources than the BBEG, but it is also something which a moderately levelled spellcaster is likely to have.
Spells such as teleport or plane shift can be used by the BBEG to escape the battle entirely. If the goal of the players was to kill the BBEG this is no good for them, but otherwise this leaves the BBEG's lair undefended for the players to loot or demolish or whatever.