Immich, a self-hosted app for photo and video backup/viewing now has support for already existing libraries in a read-only mode.
I have not tested this yet, but this was actually the missing feature I was waiting for. So if you have not looked into it for a while, maybe now is a good time to do so.
That was the feature that kept me from trying it too, so now I’m going to have to find some time to spin it up. But I’ve been happy with PhotoPrism so I’m not sure I’ll change from it.