I want to add my agreement to the 6 man party size. IT does not need to be full featured companions for the additional increase in party size. For example it could be the player hiring mercenaries that go along with the 3 companions. They do not need to have conversations with the other party members and it could be woven into the story somehow. As a example you could have the mercenaries recovering from a mind flayer removal and their mind being emotionless so they have no input on the actions of the party, Then you could make it so at the end of the story their have alignment personalities that reflect to what their personality will be at the end of the BG3 story. This could translate into DLC, as now the characters want to find out where they came from and then they make choices based on the alignment they have become in the story. It add to the complexity of the battle the player could have and allows for a lot more creativity in battle with strategy.
This game feels too much like Divinity orginal sin with just 4 party members. At least Pillars of eternity 2 did not shrink down to 4 (pillars of eternity 1 had 6 party members) and let the player hiring mercenaries to fill in the slots where needed. I know its a lot more work to have 5 full featured companions in the game, but their are ways, like Pillars of Eternity 2 did, that can make it work. The player also did not have to take 5 party members in that game if he or she wanted to, it just had the option of having 5 party members (6 would be better).