Pocketbook devices mostly run on Linux, except for a few android models. I have the Pocketbook Basic Lux, and I side load manga onto a SD card for it. It has a bookstore that can be entirely ignored, and you can use it without ever connecting wifi. I use Kindle Comic Converter to resize all my manga to the same resolution of the screen to prevent artifacts when the software downscales the images, as well as significantly reducing file size. It has a very simple menu and does not come with any bloatware or unnecessary additions. It is simply an eReader.
It seems they took some liberties to do more cool action scenes that weren’t in the manga. Otherwise it seems almost the same content-wise so far. I like the way Momo looks in the manga; she seems more lighthearted at times.