It's been enough pages that I feel the need to re-mention my preferred solution: Split XP.
Larian balances BG3 for a party size of 4, allows for an increased party size, and the XP each character gets from encounters depends on party size. Larger party? Each party member levels up more slowly, auto-balancing the game (at least partly). Similarly, party sizes of 1, 2, and 3 will level up faster.
Importantly, the toggle to allow for a 6-person party should be located in Game Settings and come with a warning: "BG3's intended experience is for a party of 4!" If there's 2 extra party slots by default, then people (myself) will feel obligated to play with a party of 6.
Split XP isn't a solution fo balancing more party members, as XP is a very small component in the balancing act. You have to account for a lot of things, like spells interactions, enemy power, placement and number, loot amount and quality, and combat duration. There really are no easy way to allow for an arbitrary amount of party members and keep the game enjoyable for all settings.
Or you don't have to account for any of that and just warn people that it is not intended to be played with 6 party members.
As someone who works with customers daily let me just tell you. This. Doesn't. Work.
People don't care if they have been warned or not. If a game breaks beacuse it was built for 4 and you get 6 it will be Larian's fault. If it's too easy? Their fault. Cutscenes work wonky? Of course it's their fault. Didn't reach lvl 10 by the end of the game? All on Larian.
Even if the players get a popup window every 10min warning them the game was designed with 4 in mind they will ignore it and when faced with a bug will blame the developers.
So no, if they decided from the start we will have a party of 4 then changing it last minute is a terrible idea.