Hah, nice! I've got to say that this recent Poland was my best yet - admittedly the only game I've played so far where I looked like I might be 'winning' by any meaningful definition of the word. Usually I conquer my first kingdom and and within a decade get assassinated and lose everything to gravelkind or a last-minute swing in the elective monarchy. I usually don't have the patience to try to re-conquer the kingdom unless my heir retains a solid claim and the successor is relatively weak.
I'm still learning the game though! Which is something, considering I've already dumped like 80 hours into it.
My biggest stumbling block is definitely succession. Every time I my king dies, my heir is at like -100 with everybody (and often -200+ with the biggest duchies, just for good measure). Then it's a flood of rebellions that I can only put down if I empty my coffers into mercenary armies.
The succession for the united kingdoms of Poland, Pomerania, and Lithuania cost me nearly 2,000 gold in mercenary fees.
On the upside, my first king had like 15,000 score by the time he died. Ironically, more prestige than the Holy Roman and Byzantine Empires combined. I have no freaking idea why. Maybe they broke a lot of truces or something. Then again, the average lifespan of their rulers was like ten years.