My only solution to fulfil my wish is to change NPCs class/kit using ShadowKeeper. For example, I turned Imoen into thief/cleric, reduced her intelligence and added them to wisdom, reduced her HP, and changed her weapon proficiencies. Okay, it won't be perfect and some conversations will not match (ie. she will still be an archmage), but I don't care that too much.
Of course, it would be far easier to create whole 6 characters for *perfect* party, but that would be rather quiet, boring party to travel such a long way. I keep their total ability scores to avoid making them too powerful (again, it's easier to give them all 18 but that's not fair and boring).
I wonder if there are anyone else tried similar?