Jailbreaking allows installing custom reader apps like KOReader (https://koreader.rocks/). KOReader has lots of nice features, including top-notch EPUB support, Wallabag integration, a PDF reader that can do reflow with k2pdfopt (https://www.willus.com/k2pdfopt/), and generally lots of ways to customize the reading experience. I don't have a jailbroken Kindle but I love it on my Kobo