Here are some things that didn't go so well, and some points of interest
First, seeing this was probably the most annoying thing in the playthrough:
It's a glitch that renders some character eternally immune to assassination and imprisonment, due to faction mechanics having a hiccup. Because of my huge amounts of money I could get around truces by assassinating my war target, but this stuff meant I couldn't do it! Another king later on had this too, which meant I had to take a truce breaking penalty for the first time.
My biggest regret overall is not being able to use an exploit I knew was possible, but didn't work when I tried it here. That exploit is having medium crown authority as an unreformed pagan. It seems to have some really specific restrictions, such as only possessing duchies in de-jure Britannia and then forming England. I really don't get this one. But if you could pull it off, then hypothetically, you could reach absolute crown authority with your initial ruler! That would have been really awesome.
But on the upside, I did have one huge lucky break in this game. You guys should have noticed that my final subjugation was on Cumania. However, that leaves one large and power pagan kingdom still free: Hungary. My only option would then be to reform and use holy war, but that would mean losing my +50% retinue bonus really early on, or leaving it for last and trekking my troops much longer than I would have liked. So I thought: "maybe if I assassinate them enough, this problem will solve itself?"
And it actually did! For some reason, the next heir after the initial Magyar family was some random dude in Cumania. So I landed him into a king, got him to inherit Hungary, and then revoked it all on the same day. I also learned that simply making him a count or a duke won't work, as he'll gain independence if you do that. The workings of this game are sometime quite odd.