Thanks for those details! I came along a similar trajectory, starting when we had our first kid which happened right around the time when phone cameras got good. Which means many gigabytes of video from phones filling up monthly.
I actually ended up building something like immich just a bit more halfbaked for our home use, that could take the whole /MobileSync iOS backup and pull out anything that looks like a photo or video. That way our workflow was, back up the phone in iTunes, let the app process the MobileSync backup, confirm they are visible, and hit the delete all button on the phone. Rinse, repeat. The storage was on a beige box with a couple mirrored drives.
At this point the collection is about a terabyte, which is not extreme by modern home lab standards. My main remaining concern at this point is that the file system is just plain ext4. Even though I mirror the collection on a regular schedule, more and more I am wondering if there is a chance of bitrot since as far as I know none if it is checksumed. I would love a solution that periodically scans objects on the file system. I would love to have an easy way to tell which of the copies is the corrupted one, since with two copies in my case I can't take the majority.
p.s. and I guess one question for you, do you think I would benefit from switching to immich for part of this workflow? I'm thinking I can't just throw raw iOS backups at it so maybe I need a bit of preprocessing there, but otherwise let it take over the cataloging and just keep the underlying storage backed up.