Ah I gotcha. I'm building a product[0] for non-jailbroken Kindles and would love to solve new problems.
In your case, if I can backup your highlights (and notes) to somewhere else, does that solve your problem? You can choose your storage service (dropbox, notion, s3 etc.)
Yeah that would be nice, could try it out. But I'm fickle - before putting that effort in, ask more of your users! Also, a table of contents with all the articles in the book would be nifty.
Maybe you can send me an email once you do it, I can try it out (I'll use the chat on the website to send my email).
Also, just a heads-up: the 50$ seems like a but too much. For context, I bought my Kindle itself for ~40$ (India)
I didn't try out the preview thing the first time since setting up the email is too much of a hassle (didn't know it would dynamically preview). But I typed in an article url anyway to see what would happen anyway, and I LOVE the kindle preview. I think it's brilliant!
If i may offer some feedback, demo screens from the app/website, and some example sources outside of Twitter ( e.g. i wanted to know if it's capable of reading RSS feeds) might be useful additions to your site.
I wish i knew about it before buying an Onyx Boox Nova Air to replace my Kindle for that particular use case ( news reading, mostly through my RSS reader which is web-based).
If I create a new book for everything, it would become messy _really_ quickly.
So if it's possible to append pages to a book, I can also create a book per category and append pages as I save articles.