Debian has a package ssh-import-id which you can then run ssh-import-id-gh $githubUsername and it will ingest all the public keys you’ve put in GitHub. Should be able to easily add it to the cloud-init.yaml but I just always install and run this first.

You can also just copy the keys to the install when you are imaging the SD card.

Or use Ansible.

You should only have to backup the Postgres database. But it won’t hurt to have a copy of your compose file as well.

This GitHub issue has the steps you should use. And answers all your other questions too.