Just throwing out an idea, could you create an smb share from nextcloud. Mount that smb in the server that hosts Immich( even if it’s the same server that hosts nextcloud).
Now, in Immich add an external library. The path of external library could be the smb mount.
Consider testing this, if it works for you.
I was going to say netdata as well. But I am not sure if it shows container wise stats.