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.
They actually added this in Beta 3 for WotR.
It goes up to x3 speed. I think going that high makes things TOO fast... (You can also press the enter button during movement or the enemy turn to make things go so fast that you might as well be skipping the entire animation, and you can also use it to skip cutscenes. The game never tells you this. At least for now.)