Oof, you’d be on the road a lot. Black forest - in the very south west, Neuschwanstein - pretty south east, Berlin - very north east.
I might suggest going to the black forest, visiting all the historical castles in the Mittelrheintal (middle Rhine valley), and instead of Brandenburg gate maybe Stuttgarter Schloss etc. Neuschwanstein is basically castle-cosplay. I think in the middle Rhine valley you’ll find cooler things and it’s closer .
Hopefully it’s a European billion (million, milliarde, billion, billiarde etc) so 10^12