Frankfurt ( State of Hessen) is where I spend a lot of time but the other time is spent in the area ( look on a map) between Cologne and Dortmund; North Rhine-Westphalia. Cheaper entries, cheaper rooms but better service than the mega clubs. I prefer small and mid sized clubs. The mega FKK clubs full of tourists I will only go to Monday thru Thursday. Big fan of red light districts especially in Frankfurt. I usually stay in Frankfurt, Leverkusen, Essen or the Dortmund area. The Germans have clubs from shit holes to the fancy FKKs. GPS will get you to most places, trains/trams/bus as well or at least close enough to walk or take a taxi.
Here is a club in Holland next time you go if you want the FKK club but don't want to go in to Germany,
https://www.sixsens.eu/