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 
).