Do you not consider systemd services to be declarative/standard?
That's not really true anymore. I can understand why you might not be familiar with declaring systemd dependencies as it's a fairly new thing. Don't blink or you might miss it.
Check out the article[0] below, it just came out...in 2015.
[0] https://fedoramagazine.org/systemd-unit-dependencies-and-ord...
For complex stuff like Immich, I'll use podman. But these days with uv, even managing python services with systemd is fine.