I love playing as Poland. So much opportunity... but honestly the large-scale combat in CK3 is so cumbersome. The game would be much-improved by including the army management of EU4 or CK2. I ended up conceding my little bit of Brandenburg to the HRE just because I didn't feel like fighting a big war at the time. I took the rest of Pomerania because my sons (later brothers/uncles/whatever) had claims. I was inspired by the old propaganda posters about "We once reached to the west!" where Poland owns Lubeck. Conquered Lithuania since it was still pagan, stopped when they converted. Conquered Lwów to make Kazimierz proud (and because Kyiv is ruled by Khazars...)
I used the Bookmarks+ mod, which is SUBLIME if any of you haven't used it. It adds so much to the game, and I honestly can't imagine playing without it.