Same person answering from Lemmy because character limit.
I don't know much about digiKam, but looking at it got me interested in checking it out! One problem I have with Immich is that there is really good Android integration with its app, but poor Desktop integration (I use a web app).
For Immich, I suppose you could mount an external library and use that same folder for digiKam. Immich will store its own set of files in its data structure for the external library, not the pictures just it's own metadata as I understand it. Any changes will be reflected in Immich after a library scan. (I will be trying this option soon.😁)
You could do the same with Nextcloud by just mounting the Nextcloud drive and setting its photos folder as the folder for digiKam, I suppose. You would be losing out mainly on Android integration with the Immich app. I can't whatever the opposite of recommend the Nextcloud options enough.