Photos missing in iCloud, only thumbnails available locally - can I extract the thumbnails?
Unfortunately you are looking in the wrong place.
By default the Photos.app (macOS Catalina) stores everything in redundantly named &quot;Photos Library.photoslibrary&quot; usually kept in the ~/Pictures folder.
If you right(or control) click on the .photoslibrary file and select Show Package Contents it will open a new finder window. I have my Finder set to display columns but the thumbnails are all stored in one place:

Once inside the .photoslibrary package file they are in:
iPod Photo Cache/FXX/T####.thmb
As above. There are not real big or very high resolution but they are something.