Braunlage in the north of Germany, in the middel of the Harz national park is a beautiful spot with some pretty hard climbs. You can easily do a 100 km ride with 2000 hm. Take a look on my kommoot collection, please also see my done collection on ride with gps, where I had 3 great rides in the september sun and a little problem with a closed road.
It can be a problem planning routes in Germany because Google Maps can't show street views and Komoot is not always up to date with road surface, so sometimes you end up wishing for a gravel bike.
Climbs
St. Andreasberg