The two things that popped into my head are Immich and Nextcloud. I think Nextcloud is generally more useful, but Immich is more specifically targeted at Photos. As for how to synchronize it… Syncthing? Personally, I hate setting up Syncthing and so I don’t really use it myself anymore, but once it’s set up, it really does take care of itself. Poke the computer once a month to make sure it’s still alive, and you’re set.
You could probably host Nextcloud at one site and just have a client computer at the next site set to auto sync everything.
Been running NextCloud for a while, not for photos, but for just general Google Drive replacement.
This is actually a real problem… A lot of digital documents from the 90’s and early 2000’s are lost forever. Hard drives die over time, and nobody out there has come up with a good way to permanently archive all that stuff.
I am a crazy person, so I have RAID, Ceph, and JBOD in various and sundry forms. Still, drives die.