On MacOS, iTunes/Finder stores (optionally) encrypted backups at

   ~/Library/Application Support/MobileSync/Backup

I was thinking of creating a symlink to Dropbox, for instance and having my cloud backup there. I don't know if the backups are incremental which could be a storage problem, but that can be managed through some scripting.

Since we hold the key it's a blob nobody can get to without brute forcing it.