While it's hard to judge balance, especially when it's not clear what you're balancing against, these two house-rules both have a pretty effective point of comparison... in an older edition.
In v3.5 of Dungeons & Dragons, Wizards had the option to something like what you suggest. They could opt to prepare only part of their spells and keep the rest open, so they could be filled in later. Wizards were considered one of the most ridiculously powerful and flexible classes in the game.
What you are proposing is even more powerful, because the Wizard at least required at least 15 minutes of rest to prepare a new spell in an open slot.
In my games, I've seen this feature used by a few players. (Most opted to just prep everything at the start of the day.) Here's what happened:
Gameplay slowed down. Whenever my players would run into a problem that they could conceivably solve half an hour later, the Wizard would go down his spellbook and look for possible solutions that they could prep. Wizards tend to have a lot of spells in there, so this would take a while. Lots of banter would ensue over which spell the Wizard might prep. Half the time, in the end no spell was prepped and a different solution was chosen altogether.
Players would look to the Wizard even more than normal. A Wizard who preps everything at the start of the day is incredibly flexible. A Wizard who can prep anything once you know what you're up against is just unreasonably flexible.
The Wizard would ultimately stop using the feature because it's a huge mental load. As the spellbook grows, the number of things a Wizard could do in a given situations rises very rapidly. It was doable for a few levels, but it quickly became too much.
In the end, most players chose not to use the ability because it was both too good and too cumbersome. It would take too much table-time away from having fun and made a lot of encounters too easy.
You are likely to experience the same things as level goes up; the caster's options will go up rapidly, which increases their cognitive load and makes them much more powerful. If you think picking what to do in combat is bad when you have 5 spells to pick from, imagine how bad it gets when you have your entire spell list to chose from.
One of the reasons 5e cut down on how many spells you can cast at a given moment is because combat flows a bit more smoothly that way. And, like I said in my comment, there is a reward in actually thinking ahead now that you will lose. It's not your fault (or problem) that your players aren't gathering information about what they will be facing.
(As for hounding players to select spells; I always rule that after a long rest you prep the same spells as the day before unless you tell me you want to make changes. Most players have a default loadout that they use most of the time anyway).
A lot of wizards are inherently imbalanced in WFRP from a combat perspective. This is supposed to be counterbalanced by Tzeentch's curse, but in my experience, that seems to trigger less often than it should. Even an Apprentice wizard can hit harder and more accurrately than a fighter of equal experience, thanks to their magic missle attacks, and it only gets worse when they become Journeymen. I am familiar with this problem, though my party usually doesn't focus on tactical combat. I know of no "simple" Slaughter-Margin like solutions, because the imbalance in combat capabilities is big and I presume you still want the other characters to stay relevant. My solutions, some aspects tried and some theoretical:
Smart enemies
Wizards can cause damage easily, but the enemies should know that, or find it out early in the fight. So they concentrate on the wizards. After the first or second crossbow bolt most wizards are barely standing on 1-2 Wounds, which is a very good incentive for the rest of the party to defend them, and for them to frantically try to stay alive. A few goblins with bows pose minimal threat to any fighter in medium armor, but they can knock a wizard down a few pegs in a single sneak attack salvo. So enemies being reasonable and not stupid, and trying to take out major threats can do wonders for combat encounters. I used this several times with a Journeyman Light wizard and his insane Eye-laser.
Not-smart allies
Wizards are supposed to be feared and hated in most of the Old World, so play with that. Have allies or bystanders around, or simply set up circumstances where blatant and flashy magic use would cause problems. This incentivizes planning on part of the players and makes them think twice about using magic to solve all problems. Because if you are allied with Sigmarite zealots while fighting orcs, subtle magics might remain unnoticed, but a burning missle of death would cause questions, which could lead to burning death on part of the caster. I also did tries out this one, the end result being the party slaughtering both the attacking goblins and the caravan they wanted to save. It was pretty interesting combat- and roleplayingwise as well.
Hordes of mooks
Most wizards don't get real AOE attacks until later careers, and while mooks pose only moderate threat to a well-equipped character, most wizards would have trouble with a lot of zombies/skeletons coming their way. A single one, or even half a dozen pose no real threat to a wizard, but a dozen or two would pose enough of a threat. The important part is that they are unable to blast enough of them in time, so they need tactics and strategy to win. Hordes of mooks are one of the things that armored fighters with Parry and Dodge can handle much easier than wizards, at least in my experience.
A boss with adds
A wizard might be able to cause a great deal of damage, but anything that can take it due to armor and high Toughness (Rat-ogre, Minotaur, etc) can stall them for several rounds. If said boss has minions of their own, especially ones who are smart, combat gets interesting again, as the wizards have to choose between blasting and trying to survive, while the others have to balance themselves between defending the wizards and killing things. I sort of tried this, though the wizard opted to go full blasting and ended up burning a Fate point.
What not to try
Wizards vs wizards. Because rocket tag is not fun or tactical, and the winner usually comes down to luck. It is very much in-line with WFRPs spirit, but it doesn't make for interesting or tactical combat.
Best Answer
There are 6 seconds in a round, but...
On page 189 and 190 of the 5e Player's Handbook it states:
The rules are pretty clear; a combat round lasts 6 seconds, and you can't speak very long during your turn. The players telling you that you can't perform long speeches or other roleplay during your turn are broadly correct, according to the rules.
However, the DM has broad discretion to alter the rules in order to make the game more fun for the players. This is often referred to as Rule 0: The responsibility to make a fun game, and the ability to selectively ignore or edit rules to facilitate that.
It sounds like you're looking for a game where the players can have actual conversations mid-combat in a more cinematic style. It's a common trope in other media for the hero and villain to lock swords and speak to eachother for a moment while the battle rages on around them, and it can be satisfying to emulate that sort of thing in roleplaying.
If you're the DM, you can talk to your players and make sure they want the same kind of game that you do. If the group all agrees that a cinematic style where players can have short conversations in a single round rather than just a few words, you can implement that immediately. If the players prefer a game where combat and lengthy rleplay don't mix, then it's unlikely that they will take advantage of such a rule change, even if you introduce it.
If you're a player, then you should talk to your DM. Let them know that you want more opportunities to be able to roleplay in combat. Tell them exactly what you say in your question here, and let them know your concerns and thoughts. If they agree, then they can make a rule change that lets you roleplay like this in combat. However, if the other players really don't like having that much roleplay in combat, then you might have to limit yourself to the "brief utterances" that are allowed by the rules.
One final thing you might try is to stay in the spirit of the rules, and try to intimidate, deceive, or otherwise hoodwink the enemies using shorter phrases. A simple "Cower, enemies, the cavalry is coming!" fits well within a 6-second round, and is possibly grounds for an Intimidate check to get your enemies to run away. You might not be able to have all the roleplay you want in a single round, but if you limit yourself to smaller outbursts, you can accomplish a lot of the goals you list without changing the rules.