I've been keeping my eye on Immich for a while and keep waiting for a stable release to try it out, but that hasn't happened yet. I'm also dreading having to setup proper backups if I were to switch to this over Google photos. My current solution is to backup critical homelab things to Google Drive automatically but I'd want a proper off-site backup if I were going to self host all my photos.

So you use Google Photos and backup to Google Drive? Sorry to say, but if Google ever decides to deactivate your account (which can and will happen for any reason, real or imagined), you lose everything.

A stable release is only a couple months out. Maybe do a Takeout until then, and put it in S3 Glacier, or similar?

I quite like their backup story. Immich has one folder that you need to backup. It stores file and dump of db that immich does on schedule. I don’t care that much about db dump but backuping photos os very easy