I use the sync a few books at a time. I have to sometimes switch between devices because of lighting issues and my Voyage does not leave the house. Thus, if I am reading a Baen book purchased from Baen, once sent to the Cloud, I can read it on any device linked to my account. When I'm done with the book, I zap it out of the cloud unless a family member has requested to read it (they all have kindles linked to my account).
Amazon doesn't care what you have emailed to your device. However, I can tell you from experience, clicking through 700 books is a pain compared with using Calibre and sideloading.
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