My solution to that very problem is to just use a third party app to sync to my NAS. I can't use Time Machine since my Macbook Pro doesn't have my full library

My solution to that very problem is to just use a third party app to sync to my NAS. I can't use Time Machine since my Macbook Pro doesn't have my full library downloaded to it (when will Apple ever start with 512 GB SSDs in their laptops...). I have iCloud Photos and pay for it, but I want to make sure I have a local backup too so I use NextCloud to sync to my NAS (which is running a NextCloud instance). You could also use the Plex app, or there's things like Resilio Sync that can auto-upload photos. I'm not aware of an open source solution for auto-upload other than NextCloud (and possibly OwnCloud, but I don't use it), but I wouldn't be surprised if there was one somewhere or an app that did auto-backup to SMB/CIFS/WevDAV/whatever