Github login does not help much... devs are on github, not on random forgjo instances. That's where they see your project. Github is also where they put their fork of your project when they play with it. They will write comments using github markdown and won't care whether that renders correctly or not in your forge.
And it is where they will report issues and open a PR. It is annoying, but it is how it is. When you ask them to open the PR elsewhere they complain sinde they need to set up an account there and copy ssh key and similar things. You need a very dedicated contributor to go through with all that.... especially if it is just a few lines of drive-by fixes.
Any of the many immutable distros (vanilla os, fedora silverblue, bluefin, aeon, endless os, pure os, ...) will all obviously work.
Most of your customizations will live in your home directory anyway, so the details of the host OS do not matter too much. As long as it comes with the UI you like, you will be mostly fine. And yku said you like gnome, that installs many apps from flathub anyway and they work just fine from there.
For development work you just set up a distrobox/toolbox container and are ready to go with everything you need. I much prefer that over working on the "real system" as I can have different environments for different projects and do not have to polute my system with all kinds of dependencies that are useless to the functionality of my system.
NixOS is ofmcourse also an option and is quasi-immutable, but it is also much more complicated to manage.