FYI, user
hollindale on the Kindle subreddit posted that he made a Tampermonkey script to mass download your books:
"If anybody has a lot of books and wants to download everything in one go before they take the option away, I wrote a Tampermonkey script for myself that will go through a single page and download everything. You have to click through the pages, but it saves you clicking the same thing 25 times for each page and forgetting where you were in the list.
https://github.com/chrishol/greasemo...indle-books.js"