Backing up and compressing Photos Library.photoslibrary I am backing up Photos Library.photoslibrary using rsync.

In general my backup philosophy is to keep things only if they cannot be automatically regenerated. Can anyone recommend items inside this package which do NOT need to be backed up (thumbnails, etc.).

If you delete these items from a Photos Library.photoslibrary and then open in Photos.app, will those items be automatically regenerated? And is there documentation of such from Apple?
You don't need the thumbnails.