@Gargron FWIW, you can move the repo and turn GitHub into a mirror.
GNOME did it before moving to GitLab, and it did improve contributions. (The recent move to GitLab majorly improved collaboration even more — but that's because they had relatively antiquated systems that still needed to be used beore.)
But, basically, you can move and keep the project on GitHub as a mirror. If you do so, I'd suggest having prominent notice in the README and links to the main repo in the README & project URL.