How to automatically back up and archive my photo library?
iCloud Photo Library would achieve a very similar end result.

In Photos, click Photos > Preferences, choose the iCloud tab and enable iCloud Photo Library.

If you turn on Optimise Storage, iCloud Photo Library will automatically manage the size of your library on your Mac. All of your original photos and videos are stored in iCloud while thumbnails are kept on your Mac. You can download the original photos and videos when you need them. 

Note that your photos would no longer be stored in any backup you make, but iCloud Photo Library does keep deleted photos and videos for about a month.