I think after the first year you’d have to populate the “Premiership” with the winners/top 2 from each division and the next top X overall scores regardless of division. After the initial sorting, you can have top 3 teams in each division advance and the bottom three get relegated, but you need a good sorting for the first year otherwise you have disproportionately arduous paths to get to the top division.
Is this going to be a keeper league of some fashion?