I'm using iCloud Photo Library to store all my full size photos and keep everything synced between my laptop and iPhone. The laptop has a small SSD, so I use the optimize space feature to keep only low-res photos locally. This all works great, but I want to have a backup of the full-res photos on an external disk, in case the iCloud servers fail. Time Machine will only backup the low-res photos in the local library. I want to continue using the local Photo Library without having the external backup disk connected all the time.
One possibility is to create a second macOS user account, set it up with the same iCloud credentials, disable optimized storage, set its Photo Library location to the external disk, and login to this account weekly to do a backup. But this seems sort of clunky. What is the preferred "Apple way" of doing this?