I know this is not exactly what you are looking for, but the Usenet indexer https://scenenzbs.com/ is specialised in German (+ German/English Dual Language) releases. Registration is open. They have a lot of stuff, not everything if it's older or rare, but enough. For new stuff it's perfect and there is basically everything.
Let me know if you need help setting it up, this is the best way to get German content if you don't have access to a private tracker.
PS: If you are looking for a German dual language Radarr/Sonarr setup I made a guide for that (works for public torrent trackers too, but as you said - there isn't a lot of German content on them): https://github.com/PCJones/radarr-sonarr-german-dual-language