And also consider moving some of your repos to Forgejo. I’m running it for more than a year now and it is by far my favorite service. Way faster and essential features do not require monthly payment (branch protection for example). It can easily run on a Raspberry Pi 4 1 GB RAM.

Use Docker Compose and put Caddy in front of it for HTTPS. For backups the easy way is to just git pull your repos via cron on some remote systems. Or use syncthing to also move the server configs over. For the runner, 1 GB RPi 4 should be fine for many situations. It can compile and run many Rust/Python tests fine or build static sites. You could also setup an old x86 next to it (this is essentially what GitHub Runners are too: old x86 cpu’s).