You know, I have the hope that @forgefriends will help us here with F3.
Basically, providing a service that allows for communication between different (git) Forges.
I'm applying for UI/UX in this regard (with Forgejo) at NLnet at the moment. Ideally with deliverables that can be reused.
What I've seen so far is some people putting a link to their new home (e.g. Codeberg) in the README and archiving GitHub repo.