I usually convert my CBR/CBZ files with CBR Converter (on Windows), a program that converts the image in the CBR/CBZ (given they are all packages), being them PNG/JPG/whatever to WEBP, so I almost don't lose quality and the size gets shrunk.
But I noticed only CBR/CBZ with PNG/JPG images packed get recognized by Nautilus and have their thumbnails generated - those ones converted to WEBP just have a standard icon on it.
I even tried this solution (How to preview DDS and WEBP images on nautilus?) and it worked great for WEBP files in general, but I didn't got much success messing with CBZ.
Is there any way to create a script to make it possible?