If you only want a backup, go for syncthing. You must have a lot of images if you can't store them all on a phone. Videos can be a problem though.

Best option I can think of is to save your videos in a different folder than the images and use syncthing. But set the settings such that if you delete it on your phone, it won't be deleted on your server. Then selfhost jellyfin and you'll have your own "netflix" with your own videos!

Thank you for this idea! I'll implement that myself!