I think it was a good idea to take the challenge to do this "manga Style", but it didn't quite work out. You would have to cheapen your process a little to get the right effect. You would need to make Roxy's hair almost completely black, her skin nearly white, and use some simpler shading. I also think the shading should look like the 'tone' textures manga artists use in their printing format and also her hair could probably use some thin lines like the stroke of a pen. The radial 'woosh' of the butt cannon looks authentic though.
On the other hand: my advice comes from a 'how to draw manga' book I read 10+ years ago, individual artist have their own styles, and I have no idea what the original style of Nozomi is. So take my advice with a grain of salt. There are probably manga artists who put as much detail into shading as you and even if not, making a picture of higher quality than it's supposed to be isn't really a bad thing.
Out of curiosity, which did you do first? The colorized or monochrome version? I like the work you do with colors and think it's a real strength of your style. The black and white version isn't bad, but it isn't as vibrant as some of your other submissions. It's cool that you tried both versions to be thorough.
This concludes my 4 paragraph review of a butt joke. Stay ambitious!