Another possible solution to this problem occurred to me.
As I understand it, I have the ability to export all Photos and end up with a big zip file. Assuming (and this is a big assumption) that the folders and folder names are preserved in that zip file, then it could be possible to:
- Export to zip file
- Do an offline batch resize of the photos
- Create a fresh zip file
- Pay groups.io a one-time fee to wipe out the current photos and replace them with the contents of the zipfile
Ownership of the files would likely be lost (I assume the zip file wouldn't contain that information), but it could be probably be restored by group admins on an as-needed basis.