The Android app (and iOS, too, probably) already has the ability to back up photos to Proton Drive. What would be really helpful, and for feature parity with Google Photos, is a "Free up space" option that would simple delete photos from the device that have already been backed up to Proton Drive. You could do Google Photos one better by having an option to do that on-the-fly, as well, instead of it being a manual process.