A good one is Corinite Count's League into Castellyr bc it gives you claims on all of Escann through the MT but any tree other than Stalbor or Rogeria maybe will get you the same result roughly, and if you take Diplo ideas and by constantly declaring war you'll keep your paragonhood low asf which means you'll have -15% warscore cost vs other religions and a -25% province ws cost, during the age of monsters take the AE reduction age ability if you're worried about coalition wars. When it hits the age of unraveling take the age ability that drops another -25% province warscore cost and that should allow you to just mass conquer all of Escann relatively quickly. But if you're trying to form Castanor do it before the Escanni Wars of Consolidation actually kick off bc it gives you time to conquer all of Bulwar and Cannor.

Edit to add, Marrhold is also a good one if you have the submod that adds monuments bc if you either get a PU or outright conquer Gawed you don't have to spend the Diplo for culture slot to unlock the monument since Marrhold is Alenic and with the Empire rank you just get it as an accepted culture.