@quad @antsu Tbh Koreader is great I like it very much. Together with Kavita you can access your library via ODPS, Webdav I dunno about. But with Kavita it worked well for me.
You will notice a difference in color-rendering against the stock pocketbook-reader. It will not be as vibrant as with the stock-reader. Though that one blasts the contrast super high generally and has noticeable color-banding.
I like using Koreader in general. Gets out the way and works. Just setup for the first time is a little annoying here and there especially with getting that title screen set to the new book when you turn it off.
And about the storage problems you have... Tbh use manga that are smaller in pixel-sizing and have better compression (pretty sure you have cbz or cbr).
Most published manga I have are like 90MB at max. High Quality Manga are nice maybe on a tablet that can display more colors and generally has higher res/ is speedier loading stuff. You might be hard pressed to notice a difference on the small screen mostly. I only saw it become really noticeable when the image was smaller than the resolution of the screen
But the 6" Readers do only really do 1072x1448 (half that in color) anyway (300DPI)
The 8" ones do 1404x1872 (again half in color)