As BG3 currently is, there is no support for playing a solo playthrough because solo characters don't get any more exp. It is strictly more difficult. While splitting experience between all party members won't help a solo character once they reach the level cap, this mechanic would at least help until that point. And it will also balance the game for party sizes larger than 4, so everyone benefits!
As someone who likes soloing for the challenge and in this game to make combat feel faster, I do not want to gain xp faster than I would in a group. Why can't there just be two options, xp for 4 and xp for 6, or just warn players that it will be easier for them with 6 and keep it balanced for 4? Or just change it to allow 5 as a compromise.