To more specifically answer OP's questions.. I'm a fellow Arq user. On your mobile devices / laptop(s) you would configure iCloud Photo Library to optimize auto

To more specifically answer OP's questions.. I'm a fellow Arq user. On your mobile devices / laptop(s) you would configure iCloud Photo Library to optimize automatically so that it frees up all the disk space. On a stationary Mac of any flavor, configure it to store the entire repository locally and then run Arq on that computer to back up the entire archive to N providers or your own off-site storage. This does have the slight drawback of requiring a macOS device in order to have the full photo library and run Arq, but paired with the usability of accessing any of your photos on any device securely, you basically get everything you will ever need. This can be something "cheaper" like a used stationary laptop with external SSD attached if necessary, or you can just park a Mac Mini somewhere if you can afford it. Once it's set up, you'll never need to touch a thing and everything will be encrypted before being backed up. (Important distinction: iCloud Photo Library is not backup and explicitly says so.. it is merely the front-end UI for quickly accessing any photo on any device.)