Photos missing in iCloud, only thumbnails available locally - can I extract the thumbnails?

I have around 1700 photos that I've discovered are missing from my iCloud storage. (These are family/baby photos that we desperately don't want to lose.)

When I try to download the photos from iCloud to Photos I get the following error:

I can see full-screen thumbnails in the Photos app (and they mostly look to be a decent resolution) but when trying to export/share the file it must be looking for the original and gives this error:

Is there any way to export the thumbnails from the Photos app? I've had a look through the SQLite database file and can't see any blob data that could be thumbnails, although I may have missed it.

I'm hoping someone has a better idea than just taking screenshots of each photo individually, which is the only option I can think of right now.

Unfortunately you are looking in the wrong place.

By default the Photos.app (macOS Catalina) stores everything in redundantly named "Photos Library.photoslibrary" 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.