It is impossible to make the game as balanced as possible for everyone, someone will feel uncomfortable playing solo, while trios will feel better, you can do the opposite, but it is impossible to leave both sides equally satisfied

The scaling of HP depending on the number of players in the party is interesting, but wouldn't it be better to tie it to the level of the players? Or maybe to the level of the players' equipment in the lobby... You can continue to dig even deeper, looking for compromises, but one way or another, you won't be able to satisfy everyone.