Backup and storage are not the same thing. Storage is putting a copy of your photos on an external device, whether that be your own hard disk or the cloud, that you can then access. Backup, almost always cloud backup, is where you put your data (photos, pdfs, spreadsheets, etc.), software, and system files so that you can recover your environment in case of a catastrophe (fire, ransomware attack, computer being hit by a meteorite, etc.). Backup files can not be edited directly. The big players in the backup market are Backblaze and Carbonite.
It sounds like you are interested in storage. If you already have Amazon Prime using it costs nothing extra for photos and was a no-brainer for me. You point it to where your photos sit on your computer and it will copy everything to the cloud. Once uploaded, you will be able to access those photos from any device. There is no limit to the number of photos or the total data volume of photos. There is a limit to the number or volume, I forget which, of video files and it is pretty small. The other thing to be aware of is that if you decide to no longer be a Prime member your photos will be deleted from Amazon so don't keep those as your only copies.
iCloud works seemlessly on a Mac but how iCloud works is much different than how Amazon Prime works. When you allow iCloud to access the photos in the Photos app library it moves, not copies, the photos to iCloud. This frees up a lot of space on your hard drive but now your originals are sitting in iCloud and not locally. When you edit locally your file is downloaded from iCloud.
