Your challenge, should you accept it, is by any PoD between the rise of Brandenburg-Prussia personal union and 1800, make Bohemia end up being part of Prussian empire and have it land-connected to Brandenburg by the time of its acquisition, by 1790s at the latest. Your conditions are :
- It must not be land-connected to Brandenburg through Saxony ! (at first at least)
- If this Prussia will ever going to acquire Saxony, Bohemia must be acquired first with interval between acquisitions being 60 years or longer, the longer the better.
- Even if Saxony will be annexed, the Saxonians will have to remain as their own socio-cultural community instead being assimilated into Prussian culture at the very least.