i'm sure it's been mentioned but one downside of a party of 6 is that you'd have to add more enemies to keep balance, making combat take even longer. In a RTWP game this is a non-issue because combat is so fast to the point you need to add trash encounters. But in turned based, it can make things feel really sluggish. I already feel some encounters in BG3 are way too slow when too many creatures are involved (i.e. Shattered Sanctuary).
Turn based doesn't HAVE to be slow with a party of 6. Larian already sped it up once. They could do more to improve the AI and keep the game rolling. In Pathfinder and Solasta, which are also turn based (or can be), you can have combats with 6 party members and the battle isn't slowed down at all. I have a hard time with this being a reason for not having 6 party members because other games can and have done it. X-com can have up to 6 party members, Pathfinder and Solasta, which are D&D games, CAN have it (though Solasta is mostly 4 party members, you can gain up to 2 additional at various points in the game). You can have animal companions, etc. as well. The game CAN move fast even with turn based. It's not impossible. It's just that it is slow right now.