If your goal is to do the upgrade the easiest thing is to spin up a new vm and migrate to it. You skip the need to upgrade the database and you get a new os on your vm, which you likely need to do anyway.
Follow Move a Discourse site to another VPS with rsync and don’t copy the database (but yes uploads and let’s encrypt and certs.
If your backups are on s3 it’s really simple to freeze the old one, make a backup, and restore the backup on the new machine.
If hetzner has some kind of permanent ip that can be assigned to different servers, you don’t even need to change dns.
You want to know that you can build a new server so that if you ever have to for some reason you’ll be able to. This is a prefect chance to practice.