I've done a fair bit of tinkering with ePaper displays attached to little Linux and MCU boards, and it is not trivial. Few if any have touch overlays, the graphics APIs are usually at the level of: magic initialization happens in this opaque chunk of bit-bashing, then you get a raw framebuffer; have fun!
You absolutely could homebrew a touchscreen ePaper "slate" with a similar broad set of features, but much like the Libre laptops (Purism, Novena, etc.) it's going to be slow, power-hungry, and chunky compared to a complete consumer device like the reMarkable.
You absolutely could homebrew a touchscreen ePaper "slate" with a similar broad set of features, but much like the Libre laptops (Purism, Novena, etc.) it's going to be slow, power-hungry, and chunky compared to a complete consumer device like the reMarkable.