I tried the same thing years ago... wasn't able to find anything better than lightroom and gave up. Now there are larger SSD's I would maybe just throw a honking big SSD on her computer and set the synology to backup her PC using active backup or if you just need to mirror the folder to synology I find the program Bvckup to be really good for that.

It’s still being developed. I’m trying lazy librarian which is also being actively developed and it has no problems looking up the book metadata.