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.
I actually thought about doing this with NixOS last year, but it seemed counterproductive compared to how I self-host, I don't want to manage configurations in multiple places. If I switched everything it would likely be just as much work and then I'm reliant on Nix. Over the years I've gone from the OS being a mix of Arch and Ubuntu to mostly just Debian for my self hosting LXC or VMs. I already have the deployments templated so there's nothing for me to do other than map an IP, give it a hostname and start it.
To each their own, but I don't want to be beholden to NixOS for everything. I like the container abstraction on LXC and VMs and it's been very good to minimize the work of self-hosting over 40+ services both in my home lab and in the bare metal server I lease from Hetzner.