For years I used Barnes and Noble’s NOOK type e tablet readers and uploaded my epub format books from various sources to read. Kindle seemed to make it harder to source your own and load them to their Fire tablets.
I just read this: “Barnes & Noble is sunsetting support for some of its oldest e-readers. It’ll start by disabling the devices’ services starting in April 2024; afterward, the company will disable access to the Barnes & Noble bookstore, removing the ability to buy new books on the devices beginning in June 2024.”