Please port this OS to older Kindles that Amazon no longer supports.

Winterbreak covers most of them >2012, https://kindlemodding.org/kindle-models.html

Or Kobo, given how shit the stock firmware is and how little the hardware has changed over a considerable period of time. Should be easy-peasy.

Nickel, the stock firmware on Kobo, is pretty decent. KOReader is better, though. The Kobo is a pretty hackable device. I pair my Kobo Clara BW with BookFusion, which gives me portability between any KOReader device, Mac, Windows, iOS, and Android.

You can already easily install koreader on the Kobo. You can even combine it with Tailscale to get your local opds server