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shatheone - If it's a b&w e-ink reader that can't read comic formats directly, I would consider doing something like this:
- unzip the cbz (they're just a rebadged zip)
- use an image editor to convert the images to grey scale, on Windows I would use IrfanView
- create a PDF from the b&w image folder - again I would use IrfanView, it has an option to create multi-page PDF's from a set of images
I don't know if Irfanview could create a PDF of ~2,000 images, it might run out of resources like calibre did. In that case I would create several smaller PDF's (e.g. 10 of ~200 images each) and piggyback merge them with something like PDfsam (split and merge).
I don't know if your e-reader could handle a PDF of that size. If it can't, buy an android tablet and download a comic book app.
BR