(author here) I've also been thinking about this - I've since built out a Rust library (https://github.com/lily-mara/kindling) for scaffolding the server piece of this and I've been considering creating a Kindle client app that integrates with it. This is possible but would require using the Kindle Java SDK, which does not fill me with excitement.
That would be fantastic, although even just the way you've done this here is great. I've got a few old Kindles that would be good to convert to displays, and if I could just install a server and a client, it would take a lot of the work out of it.
No matter what, jailbreaking would be the most difficult step in the process, but the library I linked above takes a lot of the work out of it. It's entirely undocumented atm (I am surely the only one using it), but it comes with an install script you can run on the Kindle to do the setup once you have the jailbreak done.
They really ought to, it's a fantastic reuse mechanism.
Like I totally understand why they wouldn't for new Kindles, since I assume part of their ebook sales help subsidize the hardware, but if they enabled it once a device hit 5 years old or something, I don't see what they'd have to lose.
