I have started using auto backup using OneDrive and it works like a charm. The only problem is that the pictures are left on the phone after being backed up.
Is it possible to empty the camera roll or in some way mass delete photos? (Even with an app)
Edit: I stopped using OneDrive because it kept losing pictures. Switched to Google Photos and it seems to work correctly.
Edit: And now Google Photos as a button: "Free up storage" which deletes all the photos that have been backed up.