The iCloud photo backup work very well, in my experience. My phone and older iPad are lower capacity and have to keep optimized (smaller) photos local. The full size images are downloaded automatically when viewed in Photos.
As noted by another post, iCloud will delete a photo from the whole system if deleted on one device, so you may want periodic backups elsewhere. I just drag the entire photo library file (MacOS keeps the library, photos included, in a single “file”) from my Mac to an external drive. Can’t speak to the best way to do this from an iPad, but with the new external drive support, that might be easy?
As noted by another post, iCloud will delete a photo from the whole system if deleted on one device, so you may want periodic backups elsewhere. I just drag the entire photo library file (MacOS keeps the library, photos included, in a single “file”) from my Mac to an external drive. Can’t speak to the best way to do this from an iPad, but with the new external drive support, that might be easy?