The one caveat here is that, because Apple only really allows certain apps the privilege of running in the background without being interrupted or killed, I could never get the iOS version of Immich to reliably sync photos without having to manually open it and leave the app open until sync finished.

This was a few years ago, and was one of several "iOS breaks our crucial workflows" problems that motivated switching our last device over to Android, so the situation might be better now...but I've never had that problem with the Android client, so I know it's not an Immich problem.