Syncthing is an option, but it's typically for a different use case. Most people will want their photos backed up in a way that means they can delete the photos on their phone and still have a copy, but Syncthing would delete it on the NAS end as well.
There's an ignoreDelete option but the docs advise against using it[0] and IIRC the developers want to remove it.
There's an ignoreDelete option but the docs advise against using it[0] and IIRC the developers want to remove it.
[0] https://docs.syncthing.net/advanced/folder-ignoredelete.html