I think I understand now. The FedeProxy fork (which could technically also be a Gitea branch if it wouldn’t have to address broader requirements for other forges) will gradually “meld” anything purely Gitea-related up to the point where Gitea has native “federation support”. At that point the FedeProxy Golang codebase will take on a life of its own, and become a more independent app that supports ever more forges and is installed side-by-side to them (like in the case of Gitlab).
When project “independence stage” is reached you may still incorporate Gitea code, but less frequently and there’s no need for the high-effort syncing anymore. Similarly Gitea from then on will evolve their own federation capability on their own, now based on mature standard specifications that evolve too.
If I am on the right track in my line of thinking, then eventually you’ll want to strip, rather than carry the ballast of ‘dead code’.