Have you checked tailscale? a one click wireguard, free for quite a lot of devices. there is a project to selfhost the control server https://headscale.net/

I’d try to stick to the official recommendations to backup immich: https://immich.app/docs/administration/backup-and-restore/

it looks like they don’t contemplate to migrate (yet).

I just tried and It works great! may I suggest to accept multiple destination accounts? perhaps with a config file, instead of a parameter? that way can be also possible to automate synchronizing the accounts that in multiple instances. Thanks for all the work.