You should take a peak at Google photos does on iOS. It’s basically Apple’s photos app, but all connected to Google’s cloud services. You can autobackup, keep stuff mirrored, keep stuff only in the cloud, only local, a mix of both, etc. I don’t know if it’s smart enough to automatically clear up local space if you don’t interact with a file very often, but I don’t see why a 3rd party developer couldn’t build that.