TO->PRU->GER
After fighting off Denmark, I ate the crumbling Lithuania, and as Bohemia was doing their snake thing early game I had to expand south east to block access to the steppes. At one moment my armies had all broken in an unfortunate moment and the Golden Horde was carpet sieging my lands. By launching sea borne raids from Gotland and Ösel with a 3k stack, I could kill off enough 1 regiment armies to have them accept a peace where I paid tribute to them. When I later reached the black sea I could start expanding westwards.

This is how it looked just before forming Germany. Brandenburg was trolling me for the mid half of the game and postponed the forming of Germany a great deal. First they dogpiled me whenever I was at war, then they were having the most stable dynasty EVER so that I couldn't PU them, and lastly they refused to ally me so that I could diploannex them. Ended with a war of aggression. Meh.

Well, Germany. The sprawling in the north east was entirely boundary dispute-driven.

And this is how it looks after gobbling up almost all the cores, and a few more boundary disputes to the east.