They have started implementing read-only folders for Immich with the latest release. And it does actually work pretty well.
However it currently is a very manual process via the command line (and not very well documented, especially for docker) and it has limited features as it does not automatically sync new images and does not remove deleted images from its database yet.
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.
They have started implementing read-only folders for Immich with the latest release. And it does actually work pretty well.
However it currently is a very manual process via the command line (and not very well documented, especially for docker) and it has limited features as it does not automatically sync new images and does not remove deleted images from its database yet.