OP if you use Carbon Copy Cloner to back up your Mac, there is a preference to download cloud-only files when performing a backup. Theoretically (as a rule of thumb at least) your backup drive should be 2-3x larger than your internal drive so there would be more than enough space to hold your complete library in a backup. The only thing is I’m not positive how this feature handles files inside of a library—i.e. if it will actually “pull down” the cloud-only photos from iCloud Photos or if this only works for individual files and folders say in something like iCloud Drive. I haven’t tested it to be sure but it’s worth trying for yourself.
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