Is there a reason the users must see all of the historic data too? Why not just have a post-commit hook render the current HEAD to static files, into something like GitHub Pages?

That can be moved elsewhere / mirrored later if needed, of course. And the underlying data is still in git, just not actively used for the API calls.

It might also be interesting to look at what Linux distros do, like Debian (salsa), Fedora (Pagure), and openSUSE (OBS). They're good for this because their historic model is free mirrors hosted by unpaid people, so they don't have the compute resources.