How to diff Photos Library backup against original (to check for corruption)
One note of caution, dragging photos to the desktop does a lossy export and you’ll lose all sorts of metadata and photo quality if you bypass the export functionality.

That being said, there is a tool that lets you merge / duplicate / check that two different libraries have the same or different photos (as well as the super useful deduplication features for which I pay gladly alone for this software).


https://fatcatsoftware.com/powerphotos/


There is a demo version you can try - so if there’s not another method, you can do far worse than starting with this app. It’s fast, reliable, updated and supported by the author and been around for quite many years.