I think just adding 2 more party members would be a buff enough. Numbers wise, other than enemy stats sometimes, things are pretty close to dnd 5e I think? But by adding 2 party members players will have more tools meaning that things wont have to have flat amount buffs. Also I don't think it'd be detrimental because the ideal parties are 3-6, 6 is on the higher end but in dnd that is the classic party size and in past games that was your max party size.
