I tried to setup a private relay server based on Ubuntu using the latest ‘syncthing’ version and of course with settings like ‘do not join the public pool ; listen to a certain IPV4 and IPV6 ; and port and and a certain secret token.
I partly managed. However I did not find any proper description about how to setup such an relay and could not find any thing related to the relay related config file and GUI
Have you tried following the instructions from https://docs.syncthing.net/users/strelaysrv? At least some of your questions, e.g. how not to add the relay to the global pool, are answered there. I can tell you right now though that there is no config file or GUI, meaning that you need to pass all the parameters via the command line.
If you’re interested, I’m linking to my experience with setting up and running a relay server on an old Android phone (see https://forum.syncthing.net/t/errors-when-trying-to-run-strelaysrv-for-the-first-time/16818). I did have some issues with the router, etc. but I did eventually manage to set the server up successfully. Hopefully some of the details from the struggle could be helpful from a more beginner-friendly perspective (whom I definitely was back then ).