(...) I am a bit scared because the new pathfinder game will have even more classes, races and so on plus your chars will become mystic.(...)
Kingmaker isn't hard to make a VIABLE character. A solo unfair char, yes, it is hard to make. You will need to min/max to pun-pun levels, but normal is not that hard... About Mythic paths, that is good. I really miss some P&P classes on kingmaker like witch, arcanist, oracle, etc. Loved the kineticist class that they added via DLC. As for the mythic path, i an wanting a game with path to lichdoom. The unique game with a path to lichdoom that i've played is M&M VII/VIII. Dread necromancer(mod) on nwn2 doens't give fully undead template even on lv 20.
In general, i prefer vampires over liches but a path to lichdoom is welcomed since there are decades since i saw it in any game.
And no, i an not in favor of lichdoom on BG3. One thing is having a lv 18 PC Lich or Vampire. Which is fine. Other thing is having a low level PC Vampire(low level guys can't become a Lich). Almost every DM agrees that it disturbs the campaign.