Don't throw away your Kindle, Jailbreak it and take back ownership. Make your Kindle even better with KoReader and other apps. If you wanna see whats possible i recomment this Yt video. If you wanna get started here are all instructions: https://kindlemodding.org/ Don't let it scare you it seems complicated but its just a lot of (easy) steps. So definitely possible in <2h.
You can still sideload afaik. You send the epub to the Kindle email. That still works.
Yes you can, i just said i makes it easier. And you can till amazon decides you can't... Or maby they will check the email account of you kindle and close your account because of "possibly pirated books" or whatever, they decide not you... Thats my whole point
My kindle hasn't been online since 2012. Just buy DRM free books or pirate them, and then transfer via cable. None of the firmware upgrades introduced anything useful.