You could honestly just start with this design, remove a few things, add an eInk display, use some other form of storage than SD cards for the Pi (since SD cards are notoriously indurable), and have exactly what you want. You'd probably need some custom display writing code and have to write some custom X/Wayland app that draws to the eInk screen. The idle power draw of the Pi is tiny and as long as you avoid writing to the screen with any frequency, it can probably stay alive for decades. (Though whether a Pi can last that long is a good question.)