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.
It is straightforward, but so is the NixOS module system, and I could describe writing a custom module the same way you described custom Docker images.
But it works on Ubuntu, it works on Debian, it works on Mac, it works on Windows, it works on a lot of things other than a Nix install.
And I have to know Docker for work anyhow. I don't have to know Nix for anything else.
You can't win on "it's net easier in Nix" than anywhere else, and a lot of us are pretty used to "it's just one line" and know exactly what that means when that one line isn't quite what we need or want. Maybe it easier after a rather large up-front investment into Nix, but I've got dozens of technologies asking me for large up-front investments.