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

The first question would be if you have any experience in torrenting in general and private trackers in particular?

Some trackers are notorious hard to join or even completely closed, however others have open signups or application signups. You are also missing some German private trackers in your list.

Some German private trackers like the already mentioned World-of-Tomorrow or SpeedTorrent Reloaded or The Shinning allow application signup, if you are already experienced in trackers. For other trackers like Immortuous one can signup on their forums and apply for an invitation. Just visit their sites and look for an application/signup link.

Your question is too generic to provide more helpful answers.

You may apply for wot (world of tomorrow) here

All other public facing private trackers have such an application procedure. Just apply with a good record from somewhere else. If you get access to a tracker where you don't like the content, take it as a stepping stone to get into a better tracker. Work your way up. It doesn't take too much. Easiest is to get a server to seed 24/7 . You also don't want to be in every tracker. It's just not neccessary.