How I’m exporting my highlights from iBooks and Kindle (medium.com)

You should turn it into a service. I'd subscribe to it!

The Calibre client exports Kindle notes and highlights to a flat "My Clippings.txt" for all kinds of file-formats (not purely Amazon-purchased), wouldn't using that file be easier than fighting with JSON and an apparently forgotten Amazon page?

Edit: Turns out, this exists: https://www.clippings.io/