Good times.
Yes, that actually happened. Because of the stupid succession rules I can't keep both kingdoms, so I simply dissolved the kingdom and distributed the vassals among as many non-Bohemian vassals as possible. Why? To keep them from reforming the kingdom of Bohemia. I have a feeling someone will succeed with that anyway, but it's worth trying. The kingdom of Germany has medium crown authority (which prevents vassals from fighting eachother), but I found crown laws only apply to de jure land. So, endless fighting for the periphery of the german kingdom until their lands have drifted into my de jure territory after 100 years of occupation.
As soon as king Amadeus I of Poland (my Billung cousin) was succeeded by Amadeus II, I got to push a claim on the duchy of Silesia for a Billung family member. Hopefully I get to keep it a bit longer this time around. In the image below you can see my borders in yellow, and my land in grey. The grey outside my borders is what's left of the HRE.
Oh, and Martin the ill-ruler died at the age of 75. Long live Martin IV.