I’ve had a good experience with an old Kindle 3G and the Calibre program. You can get the old Kindles pretty cheap and they don’t have the built-in ads and some of the slightly useless features of the new ones.

That, or like the16bitgamer mentioned, an iPad has good options for free ebook apps without ads or tracking. I found this one called eBoox that has great usability and no ads. It’s weirdly marketed as a “cute” ebooks app, but it’s honestly better than the stock one and doesn’t actually have cats or those pictures that are in the description in the actual app.