Actually, I just found another workaround that works even better in my case. Sharing it here in case it helps anyone else. The problem with getting annotations from Amazon's
https://read.amazon.com/notebook page is that they only include annotations from books bought from Amazon. Annotations for "personal documents" (i.e. any content added to your Kindle account that wasn't bought from Amazon) don't appear, although Amazon does capture them and synchronize them between your devices. If you run the Kindle iOS app, it now has a new feature that allows you to export your "Notebook" (all the notes and annotations for a given book/document across devices) and send it as an .html file in email. (I'm guessing we have the new Kindle Scribe to thank for this.) This file opens in the browser and contains all your annotations for a given book which can be easily copy/pasted into Calibre.