Immich is one of the only apps on iOS that properly does background sync. There is also PhotoSync which is notable for working properly with background sync. I'll take a wild guess that Ente may have got this working right too (at least I'd hope). This works around the limitation that iOS apps can't really run as background apps (appears to me that the app can wake up on some interval, run/sync for a little and try again on the next interval). This is much more usable then for example, the Synology apps for photo sync, which is, the last time I tried, for some reason insanely slow and the phone needs to have the app open and screen on for it fully sync.
Some issues I ran into is the Immich iOS app updating and then being incompatible with the older version of the server installed on my machine. You'd have to disable app updates for all apps, as iOS doesn't support disabling updates for individual apps.
In my specific scenario, the latest version of Immich for NixOS didn't perform a certain migration for my older version of Immich. I had to track down the specific commit that contained the version of Immich which had the migration, apply that, then I was able to get back to the latest version. Luckily, even though I probably applied a few versions before getting the right one, it didn't corrupt the Immich install.
And the jump of getting rid of people you hate who contribute to your project and you can do little harm to, to getting rid of people you hate who are of no use to you and you can do genuine damage to (e.g. by installing a tor exit node) is a step down if you think you could get away with it.
... Why? I don't know what developer purge you're talking about, but getting rid of people running a project almost never means suddenly they'll start to get rid of users, I'm not sure why that assumption is there. Not to mention that they couldn't even "purge users" if they wanted to, unless they make the download URLs private and start including some licensing schema which, come on, hardly is realistic to be worried about...