Your first war target is Bohemia. Rival them early for the power projection bonus, then use the Enforce PU casus belli to bring them under your crown. Call in your allies — you've royal married Castile and allied the electors by now, so this shouldn't be a hard fight. One war won, one junior partner secured.

Win your opening war against Stettin (or Wolgast, depending on their alliances) to secure your first foothold.

Your first war target is Bohemia. Rival them early for the power projection bonus, then use the Enforce PU casus belli to bring them under your crown. Call in your allies — you've royal married Castile and allied the electors by now, so this shouldn't be a hard fight. One war won, one junior partner secured.