CapnZapp
Legend
This is what I mean.I would completely scrap the Birthright rules, and try to adapt the Reign rules to my game.
Instead of writing something from scratch that is only ever meant as a placeholder, a toy; you'd need something that already from the beginning was meant to stand up to scrutiny, to work on its own.
Birthright's domain rules work only because they're meant to be filtered through a DM.