Data (documents, pictures, source code, etc.) is not handled by home-manager. Backing up home.nix saves your config, but the data is just as if not more important.
Hmm, different strokes I guess. Maybe it's just that too much kubernetes has gone to my head, but I see files as ephemeral.
Code and docs are in source control. My phone syncs images to PCloud when I take them. Anything I download is backed up... wherever I downloaded it from.
Cloud sync != backup. Cloud sync won't help if you accidentally delete the file, backups will. Cloud sync won't help if you make an undesired edit, backups will.
But I can just rebuild the file, or restore it from a previous commit (or if I'm having a particularly bad day, restore its inputs from a previous commit and _then_ rebuild it).
Code and docs are in source control. My phone syncs images to PCloud when I take them. Anything I download is backed up... wherever I downloaded it from.