You're right, PRs are much more complex to mirror, although I still think it should be possible. If someobe creates a PR on nostr, it could be mirror by a PR created by the bot (which would have permissions to push to its fork). It's hard, but doable. Let's start with Issues only...
@b10c runs a backup script for bitcoin-core and libsec256k1. These are pretty useful to understand the structure of the data and what to expect. core developer Fabian also created a one way bridge to a self-hosted gitlab. se https://delvingbitcoin.org/t/gitlab-backups-for-bitcoin-core-repository/624 I remember finding a python tool that seemed to be quite active - can't seem to find that now. I'm not sure why Fabian didn't use it. Maybe it didn't meet his needs.