Lol, I can't do this with my mom. She needs all her photos on her phone and I don't quite think I'll be able to teach her how to use a third party app.I’d goure serious about deleting them and not hoarding them (not judging I just know some people feel like they can’t delete anything), just download Flickr and let it back up to the cloud in the background while you’re sleeping. Then just wipe all of them from your phone in one shot. I plug into my Mac, select them all in image capture, and delete. Takes a matter of seconds to delete
That said, it's mostly the videos taking up tons of space so those are the only things I've had to actually offload (although even that was a hard bargain, she didn't want to delete anything!). Now my life is so much easier with iCloud Photo Library and Optimize iPhone Storage. It even automatically syncs between her iPhone and iPad so she doesn't need me to do that for her.