If I can have access to everything in one party, that removes some of the more interesting choices I have to make.
ive seen comments like this a lot too - but i really dont see how this tracks. How would having 'access to everything in one party' also 'remove some of the more interesting choices'? the player still decides what choices they are making - i dont see how an increased party size would restrict a players decision making or limit their options. if anything, you would have more tools in your bag to tackle different encounters and scenarios. (id even argue youd have more tools so the player wouldnt need to rely or fallback on more of the 'cheese' or exploit like tactics currently in bg3's ea build, but thats an aside)
Having interesting choices and having the most options aren't the same thing. Interesting choices often spring from having restrictions placed on you and then having to figure out how to work within those restrictions. Having the ability to control those restrictions (by selecting your few companions from a pool) improves replayability. One of the best feelings I have when playing D&D is finding ways to solve problems when I don't have the right tools for the job.
For example: If you are trying to get into a secret guild hall, there may be many ways to approach that task. An obvious solution is to have your party face try to convince or bribe your way in. If you don't have a good face, you need to come up with other options. Maybe you can find a way to eavesdrop on the door to hear the secret password. Or you can kidnap a member and try to take their place. Or you can spend some time looking around (maybe checking maps) to try to find a secret entrance. Or you can just use force and try to bust down the door. If you always have a smooth-talking bard because the game doesn't make you narrow your party down, you might never learn about those other options.
If you always have a full skillset available, there is a tendency to fall into the habit of doing whatever seems the most obvious, which is probably the same thing over and over again and tends not to be as fun. I generally don't choose my companions based on their skillsets - I pick them based on which characters are interesting enough to want to keep around. If I end up with all rogues, that's fine - I can find a way to work with that and it has the side benefit of letting me dig deep into the rogue class and seeing how they can specialize in different ways. Some things will be easy. Some things will be hard. I'm sure I'll fail a lot. As long as they make failure interesting (instead of just a punishment), that's fine.