Immich has so many dirs and files that it will take hour/hours for any file-based program to walk that tree (on an HDD) in order to do a backup. I used to use rsync, syncthing, duplicity and had gotten it to about under an hour for 1TB library. The service has to be shutdown to preserve consistency of the backup. So that downtime was okay once per day. Still a heavy operation. Not great, not terrible.

Because of the above you might want to do one of the following:

• Use snapshotting filesystem like ZFS or Btrfs. That allows to avoid the downtime as you can backup a frozen-in-time snapshot while Immich is running.
• The above but with ZFS om both the Immich end and the backup end. That allows transferring just the delta between the new Immich snapshot and the last snapshot the backup has. This happens entirely within the filesystem so there are no file operations needed. At every point in time ZFS knows what blocks have changed between now and the previously backed up snapshot. When you ask it for that, it starts spitting data without spensing an hour figuring out what changed. That allows online backups every hour or even more frequently. The backup of my 1TB Immich instance (HDD) takes 5-10 seconds to initiate the network transfer, then it takes whatever time it takes to get to the backup machine, which is usually equivalent to transferring whatever new was stored in Immich.

So depending on how painful migrations are for you, I'd highly recommend the latter. Otherwise the former. The ZFS snapshot send strategy works for any service you may run.