While I understand the reasons for preferring a larger party, I can't say I think it would be an improvement in BG3.
EA seems to play OK with a party of four; if you expand the party size, I can see 2 problems related to the turn-based combat being used.
- the number of enemies in encounters would also increase, leading to even longer encounter resolution, when some of them are pretty turgid already. This could, perhaps, be alleviated by using fewer, but more powerful enemies.
- increased party size would exacerbate problems with party characters blocking each other, which you can't resolve because of the limitations of turn-based movement. This is less easy to alleviate, as it would mean re-working the game world to have less choke points, many of which are related to the significantly vertical nature of the game world.