I recently did a bare metal migration from Gitea to Forgejo using NixOS, maybe this info is useful if you use SQLite (which I believe is the default): the SQLite database filename for Gitea is gitea.db and for Forgejo it’s forgejo.db so I had to do a rename. Before renaming I ended up with an empty Forgejo instance. Either way I hope you figure it out in the end. Good luck!
https://forgejo.org/faq/#what-is-the-difference-between-forgejo-and-gitea more specifically https://forgejo.org/compare-to-gitea/