PINE64 were trying with the PineNote[0], sort of. It's quite a large device (10.3"), in the same size class as reMarkable/Boox devices. It's cost-competitive with those products, but it's way too big and too expensive to compare to traditional ~7" e-readers.

0: https://pine64.org/devices/pinenote/

Seed studio sells some. reTerminal is the more polished one, it has a metal case, nice buttons, and a 2Ah battery. The Xiao screen is more diy, but also has more potential options

I used a reTerminal as a bedside weather station UI recently, works rather nice

Not exactly reading devices, yes, but they are open hardware

TRMNL are kinda.

https://trmnl.com/

https://github.com/iandchasse/de-link

Someone is working on a more DIY device that can run this firmware.

What would that look like? A first thought would be a controller with storage that would 'dock' into various screen sizes and case molds?

supernote's tablets are designed to have replaceable motherboards/batteries although it's still to be seen whether they will actually be able to support that long-term.