Is there any open source (DIY) hardware that goes with these APIs? Since $400 seems a little steep for a white screen I suspect it is possible to DIY similar hardware too. Any ideas?
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.