I think more of the issue might be the eink screens. As far as I can tell, there just aren't 5+ inch eink screens for cheap.

yes they are: https://www.waveshare.com/product/displays/e-paper.htm?___SI...

A 4.37inch E-Paper in 3 colors is $24, problem is need you to program yourself (they have code sample in python, for raspberry pi), and you need a raspberry pi, case, cables, etc.

Also, these cheap epaper displays are, of course, of lower quality (slower, lower resolution) than an kindle display.

They jump up in price pretty quickly as size goes up. The cheapest 5+ inch display I found at your link was over $40, and it's about 100PPI. It's certainly not prohibitive, but certainly priced high compared to "just jailbreak a kindle" for any remotely kindle-comparable display, right? (remotely comparable in size and resolution)

I imagine the Kindle is sold as a loss leader, plus whatever economies of scale/negotiating Amazon does pushes the price down heavily vs buying a single unit from an electronics retailer

They look kind of cool, and now I'm trying to come up with a project such that I can justify buying one.

Arduino-compatible devices based on ESP32 are plenty powerful enough at a fraction of the cost.

CPU is indeed beefy for a small wi-fi chip, but the small RAM hurts. Yeah, there's QSPI PSRAM but the bandwidth is lacking as well.