i really like a party of 6. i know larian wanted 4 because of making the game appeal to mass audience and for multiplayer. in pathfinder wrath of righteous, i played the game entirely on turn-based for few playthroughs. it wasn't slow but i do need to speed up the animation.
i like a party of 6 in bg3 but i think the biggest problem would be balance and the encounter design probably need to be revisited to add more enemies, fine-tune the HP and abilities, etc. here's hoping the mod community could make this happen.
i be really willing to donate to the modder too if they did that.