Up until fairly recently, you could just drag and drop files onto the Kindle with a usb. I’ve had my first generation Kindle for almost 15 years now and it still works. Just download an .epub file, convert it to .mobi with Calibre, and drag and drop it over to the Kindle.

I have a newer one too, that I got a couple of years ago as a gift.

The trick is just disable the wifi and never let it communicate with Amazon servers. They will mess with your settings and push secret updates that remove features. For example, it could “sync” your books with your Amazon account literally resulting in you not being able to remove items from your Kindle without logging into your Amazon account on your computer and going through a million menus. It won’t let you do it from the Kindle, even if you’re offline.

But if you just never let it connect it to the internet at all, you’re fine.

Although the new Kindles now require a special Amazon software to copy files over (because of “convenience”) and it won’t communicate with the usual protocol so you can’t drag and drop like you could for the last 15 years.

So yeah, don’t buy a Kindle. at least not a new one.