I asked this recently as well. One of the answers I got then was “mega”, but I haven’t tried it.

My recommendation is not necessarily FOSS, though some of the tech stack maybe open source. Synology makes some nice NAS type systems. Software they put on the systems are pretty well polished. They support android and iPhone apps that allow you to easily backup your phone’s camera activities. Like I said, not completely FOSS but a great alternative that would allow the less tech savvy to get away from relying/trusting Google with their family photos/videos.