There is the FreeWrite [0], which seems to be targeted at just writers, which is in some respects the spiritual successor to the TRS-80 Model 100. I could swear I saw at least one other Kickstarter with something similar, but this was the only one I could find.
We used to use those AlphaSmarts to learn how to type in elementary school in the early 2000's, before our school got a laptop cart that could be wheeled between classrooms.
The DIY version would be a PCB with an ATmega32u4, an SD card slot, a similarly-sized LCD screen, and a lithium-ion battery. You'd attach a portable USB keyboard of your liking.