Use the encounter building guidelines in the Dungeon Master's Guide
Encounter building tools can be found in the Dungeon Master's Guide, page 81 on. They can also be found in DnD Beyond (which can also do the math for you).
For example, an encounter against ten homunculi would have an adjusted XP value of (2.5 × 10 × 10XP) = 250XP, where 2.5 is the multiplier for 7-10 hostiles, 10 is the number of the homunculi, and 10XP is the experience value of a single homunculus. For four level 1 PCs, this encounter would qualify as Medium.
To get the XP value for monsters in an encounter, you can use the CR to XP table on Monster Manual page 9. Each monster's XP value is also included in its stat block. CR 0 monsters are worth 0 or 10 XP depending on whether they have a proper attack or not.
The slightly modified encounter building scheme of Xanathar's Guide to Everything (also available in DnD Beyond) presents a simpler, not quite equivalent form:
For low challenge ratings not appearing on the table, assume a 1:12 ratio, indicating that twelve creatures of those challenge ratings are equivalent to one character of a specific level.
Remember that while the encounter difficulty calculation is a useful tool in normal situations, it's always the best to weigh the output against the strengths and weaknesses of the particular party you are placing the encounter against.
1. Treat healing/round as damage/round for calculations
While the DMG doesn't give a clear answer, Spellcasting's effect on CR (DMG p.281) states:
\$\begin{array}{|l|l|l|}
\hline
\textbf{Name} & \textbf{Example Monster} & \textbf{Effect on Challenge Rating }\\
\hline
\text{Spellcasting} & \text{Lich} & \text{See step 13 under "Creating a Monster Stat Block"}\\
\hline
\end{array}
\$
Where Step 13 states:
Innate Spellcasting and Spellcasting. The impact that the Innate Spellcasting and Spellcasting special traits have on a monster's
challenge rating depends on the spells that the monster can cast
Spells that deal more damage than the monster's normal attack routine
and spells that increase the monster's AC or hit points need to be
accounted for when determining the monster's final challenge rating.
This tells us that spells in general should influence CR (Unlike some other monster features), it's just lacking by how much.
We can however reverse engineer a monster in the book to see how its CR is affected by spells such as these, by determining its CR before taking into account healing and other beneficial magic, and comparing it to its actual CR. Let's take this Couatl into consideration. Most of its spells are healing and damage mitigation, and it gets a large amount of them, leaving it as a good starting point. Breaking it down based off the Monster Stats by CR table (DMG p.274), we have:
Defensive CR: With 97 HP* the Couatl lands it in the CR 2 category. Its AC of 19 is 6 higher than the expected CR, leaving it Defensively CR 5
Offensive CR: At 10 damage/round, Its offensive CR starts as a 1. Its attack bonus of +6 is 4 higher than the suggested, leaving it Offensively CR 3
Averaging these out, the Coautl comes to CR 4. This is without factoring in any amount of healing or beneficial magic cast.
The next step is to see how far we can deviate from this, to see if magical healing increases CR on top of normal calculations. The most apparent starting point is determining if we can raise it's effective HP based off of healing magic, and still stay within the confines of it being a CR 4 creature. Looking at the HP category it fell under, we see a range of 86-100 HP. Any more and we have the potential to bump up it's CR. Seeing that the Couatl is already at 97 HP, this means an increase of any more than 3 effective HP will cause the Defensive CR to increase to 6, increasing the Couatl's total CR to 5.
Since this is not the case, it can be inferred that healing magic does not affect the Defensive CR of a monster.
Now we see an example where a creature's CR is affected by its spellcasting. Take the Acolyte. With a defensive CR of 0 and being 27 HP below the threshold for increasing (Eliminating again the possibility of healing affecting Defensive CR, as using every spell slot to heal only achieves 24 points), the Acolyte needs an Offensive CR of 1/2 to meet its average CR of 1/4. This means 6-8 damage/round. Given it does 2 with a melee attack, 4 with it's only damaging spell, and heals 6 HP with a Cure Wounds, the Cure Wounds is the only option to put the offensive CR in line.
No examples given in the Monster Manual give creatures with the Aid spell, but due to it being a spell that's cast before combat begins, it stands to reason to work like Mage Armor (DMG p.276), in this event granting HP to go towards Defensive CR calculations (As there's next to no difference in a creature with 120 HP with Aid than there is with a creature having 125 HP without Aid). Whether it affects more than one creature (Potentially raising the effective HP boost to 10-15) is detailed below.
*Doubling it's effective HP, as the book suggests you should do for a CR 4 creature with immunity to nonmagical bludgeoning, piercing, and slashing damage among other immunities and resistances (DMG p.277), brings its Defensive CR to 10 after calculations, leaving the Couatl CR 7 by these notes. Why this step was omitted I don't know, but is beyond the scope of the question.
2. There is no clear answer
The only two Monster Features (DMG p.280) that revolve around area effects, the Breath Weapon and Death Burst, operate off the assumption that two creatures will be affected, so this is the best baseline to go by. It's unknown whether the caster should be included in the two creatures, or it should be the caster plus two creatures (Since neither given feature is sufficient to determine if the intent was to affect two creatures or affect two other creatures), so some discretion is needed.
Best Answer
If you have problems, disadvantage on attacks isn't one of them
The portion of the ability you're concerned about is roughly equivalent to everyone having disadvantage against your monster, always. Let's compare this to something the Dungeon Master's Guide has guidance on: Superior Invisibility.
According to the Monster Features table, a creature that can be invisible as long as its concentrating effectively increases its AC by 2. Your ability is sometimes better than invisibility, and sometimes worse.
In fact, that last point is crucial if you're playing with flanking rules. Characters who do their combat up close will be affected by Exhaustion Pulse, but are likely to be able to gain advantage through flanking, and will circumvent the disadvantage entirely.
I think, from this comparison, it's not unreasonable to say that Exhaustion Pulse should be close in value to Superior Invisibility. That means it will increase the effective AC of your monster by something like 2 or 3. This is not, on its own, unreasonable - the existing CR 30 monsters that we have as examples both have AC 25, which would be about on par with your monster's AC.
Normally I'd be concerned about stacking Magic Resistance on top of that, but lo and behold, both other CR 30 monsters have that (or a better version) as well.
Look at the rest of the package
There are a couple causes for concern, however. Disadvantage on saving throws is huge, depending upon what other abilities your monster (or its minions) have. If your estimate of 263 damage per round relied on saves, for example, then Exhaustion Pulse has the potential to drastically increase your damage output. Similarly, if it has access to must-save abilities such as the feeblemind spell... Well, that'll make short work of any of your player character spellcasters.
It's also somewhat concerning that the effect of Exhaustion Pulse lasts for 3 rounds. That's unusual, to say the least: typically, abilities like this will either last for a single round, or until the target succeeds on its save. It's important to note that if a target failed once, it's nearly guaranteed to do so a second time, since it has disadvantage on the save against the legendary action.
So the likely play pattern is to use your legendary actions for other things for the intervening two rounds, then on the third round do the pulse again while anyone already under its effect is likely to fail. That's going to be both frustrating and more powerful than the effect looks on its face.
Is there a better way?
I think you'd do well to look at examples of similar themes here. There's a couple ways you could intend the ability to play out, and in my opinion your wording isn't great for either of them.
This ability is a challenge to be overcome
If you want your monster to seem horrifying and unbeatable at first, but for the tides to turn in your players' favour as the battle goes on, look at the Frightful Presence ability dragons share:
Wording like this makes it less likely that this is a repetitive annoyance, and more likely that players can actually plan around it.
This ability represents inevitability
If you want a drawn out combat with your monster to be effectively futile, you've chosen the wrong condition to impose. Your Exhaustion Pulse should actually be imposing the actual exhaustion condition.
The first level of exhaustion is a minor annoyance, but anyone who understands the mechanic should be instantly terrified if they see a monster causing on-demand exhaustion. Playing the long game against this thing would be a terrible idea, and it means being in its presence isn't just problematic; it will kill you, likely within a minute or two, without ever touching your HP.
You need to be quite careful about this, though. Exhaustion is a nasty mechanic, and the best example I have of an existing monster that uses it in this way, the sibriex, has a number of safety measures to keep it from getting out of hand.
On the other hand...
This is a CR 30 monster. You players should expect it to get out of hand.