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.
Splitting XP isn't a solution for 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 quantity 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.