Basically what @The_Red_Queen says above. I don't want party size to be tied to difficulty levels. People who want to play on harder difficulties shouldn't be restricted to 1-2 party members, and people who want to play on easier modes shouldn't have to play with 6-person parties. (or vice versa).
Larian should implement a party of 6 option (hidden in game settings, with a warning that this is not the intended BG3 experience) without changing anything else, or Larian should scale xp such that parties of any size (1 to 6) see similar levels of difficulty due to leveling up at different rates.
Any other options (making a party size of 6 the default, tying party size inversely to difficulty level, making 2 separately-balanced games for party of 6 vs party of 4, etc) are a combination of too much work and/or would make the game experience worse for more people than it helps.