I have used Google Photos in conjuction with a local library of images.
More so I could easily browse all images online.
But now I am transition to only using my local library. It is located on a Synology and they have a Photos component, so I have still access to my images everywhere. And I do also have an online backup at another location.
So I have made a Google Takeout request for my images (246 GB data).
And I would then find out if I had images only located in Google Photos.
Made two list of all files under my directories.
Manipulated them to filter out the file name.
And used Easy Data Transform to find those files only in Google Photos.
Output was a text-file with the full path to the image.
This I could use to copy the files out, so I can consolidate my local library.