And Phabricator, the most underrated software in history.
In Phabricator, you create the repo, you turn off its access URLs, add URLs for the repo you're mirroring, set them as a mirror source, and then turn the access URLs back on. This is about a 5-10 minute process per repo and it's all clicking around in a UI.
and it gets all f*cked up if you started a comment and didn't close the input. among many other UI stuff (not sure if the UI has improved or if any of it is configurable btw)