Recently I decided to switch from exporting yearly my whole library (375 GB minus full-text-search.db - 78 GB), through Calibre built-in feature, to use FreeFileSync with an external SSD enclosure, thanks to the sound advice given by honorable MR members.
Now I just mirror the local file changes and found it to be way faster than I thought it would! As for cloud backup, this would be awesome but...