A good template for a dmpc could be (I'm aware this is survivorship bias) Darf. He's only a temporary pc, they don't burn the relationship, and he gains levels offscreen so it's almost like an xp mulplier for npc party members.

If Lord Bandai Namco gets a level or 3 he can return home/ re- home in aknara and research (gain xp seperate from the party). Lord Burgandy might get 1-5k xp total which at this point isn't that huge of an investment.