I do the backups from my Mac. Photos on that Mac is configured to download all of the photos in iCloud Photos. It is the source of truth for my photos. The photos are stored in a library directory as discrete files and the backup software has no problem backing them up.
It would be trivial to restore the whole set of photos. It is more complex to restore specific individual photos as you would need to know the file name of that photo.
If you want to generate an alternate photo store you can export some or all of the photos but that is not necessary for backups.
Seems like as long as they all fit, that should work. But does require a non-iPad/non-mobile device with enough storage, as iPad/iPhones don’t have backup mechanisms besides iCloud.
I don’t have a Mac laptop anymore, and my library was starting to get too big to fit on any one device. That’s when I realized they had no other way of accessing/moving/transferring/backing up, so I moved it out using some special photo backup software before I got stuck.
It would be trivial to restore the whole set of photos. It is more complex to restore specific individual photos as you would need to know the file name of that photo.
If you want to generate an alternate photo store you can export some or all of the photos but that is not necessary for backups.