No mods required whatsoever, just get your ebooks in whichever format you can and then use https://calibre-ebook.com/ to convert them to .mobi and copy them over via cable. If possible, avoid pdf's - they can be converted, but it often looks shit. EPUB on the other hand converts with zero loss, and that's the most common format on libgen.rs and other ebook sites. If MOBI is available right away, just go for that of course. Conversion takes mere seconds though.
Since I don't buy any ebooks from Amazon directly, I've set my kindle to flight mode right after the initial setup and haven't switched it back on in over 4 years.