I’d look into the git-maintenance’s prefetch task. From what I understand, that is more or less what you are looking for. Then just run any old http(s) server and clone them from that https://git-scm.com/docs/git-maintenance

Nixos

Pros

• Delarative Config
• largest package repos

Cons

• poor documentation
• cli and package management is in limbo with unstable flakes/cli