A podman quadlet would be a great way to manage the Immich container.
Keep in mind your system will already have the plumbing for podman. So it's not as bad if you're averse to using docker.
It'd be managed as a systemd unit. IMO its a better method than flatpak.
Maybe this
Although it looks like the nasty docker bug link in that thread is fixed.
So maybe ro mounts can mitigate the problem.