Shoehorn into fixed directory
“you will need your albums to reside in pre-designated areas of your NAS storage (you cannot tell the app to ‘index from X location’ easily)”
This is the reason I don’t use Photos. And this is what I was hoping would change in DSM 7. In fact this was the app that was driving my update. If this ludicrous limitation is not fixed, I’ll have to look for another alternative to Google Photos.
Actually, they have added a function to add non Home/Photos folders. It is simply not very obvious.
You should be able to set up a conditional folder and choose a location like this
Otherwise, you could install Photos app on the source NAS from where a hyper backup is sending photos away to the backup NAS. Hybrid backup allows users to choose any folder to back up.
I hope this helps.