Yeah, the (stock) CBZ reader is... painful on Kobo ^^.
Plus, 32bpp, so you get a "nice" grayscale -> RGB32 -> grayscale roundtrip for no good reason (well, no, there's a "good" reason, it's called Qt5).
You'll *probably* get something rendering closer to the Kindles on KOReader

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But, yeah, that's much closer to what I'd expect those to look like in person

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(On the scanner sidebar: the flush screen probably helps the PW4, too, bringing the screen "closer". Conversely, the Libra's "handle" probably doesn't help, pushing it a bit farther away).