Just posted about a very nice club close to Osnabrueck in the more northern part of Germany called Zeus FKK club here:
https://sbf-sg.social/showthread.php?t=1050780. I also looked at the FKK clubs in Hannover like Club König, however the reviews on local forums were not very encouraging.
In short Club Zeus was an amazing experience, if you are in the area it is really worth to try.