Which kobo did you use? If it’s one of the color ones, it’s going to be much more grey than B&W e-readers due to the extra color filter layer. All the color e-readers have this drawback and rely on the front light to improve contrast.
If you don’t need color, just go with the latest gen of a black and white reader, which should be using Carta 1300 I believe, with the software you’d like. Could also go with an older generation if you want to save a few bucks, such as carta 1200, all you would sacrifice is a bit of contrast and some responsiveness.