It would make more sense if you put your requirements on GitHub as an issue. It's best not to summarize everything in one, you can't work with that. https://github.com/thunder-app/thunder

No the software in this case is a classic forum software and stands alone.

I have summarized here what I have found out so far, but is currently only available in German https://handbuch.rollenspiel.monster/lemmy/communication/

I think my next one will be a https://frame.work There is also an overview of supported systems: https://frame.work/en/en/linux

Since I do not want to use discord I can only search in forums, on discord instances you are also always banned if you look for players there but then do not use them to play. Unfortunately, the forums die out or there is only searched for DnD :(

What is FLGS?

I also like it very much.

My first thought "who is this?"

On the PC I currently play Outward as P&P DnD(which I personally like less and less), Pathfinder(the better DnD xD), Cyberpunk is currently replaced by Los Muertos, Aborea and Butterfly Aspect.