Local backup of iCloud Photo Library 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?
@Elliott: Do you have the option of creating regular backups of your external disks? If so, you could symlink your photo library to the external source, backup that source however often you like, all the while using your full res photos instead of the low res versions. The photos would not be stored on your Mac hard drive (thus freeing up that space), but would be stored on backed-up external drives instead.

I've successfully done this before using both iPhoto and iTunes (among other things), and now regularly move all my important things to external drives for safe keeping. Backups of your backups is key, but drive space is cheap!

Let me know if I can be of further assistance.