just some suggestions
If the manga's raw didn't have color, don't add color, it adds a fair amount of size, its better to convert every raw to grayscale work on it that way, and if you want to take the time, do a 32 16 8 and 4 color (this is just grayscale) test with dithering. this will tell you how many colors you need to make the image look like a full color image with the bare minimum. if you have fantastic raws, you can do a black and white 2 color image, but getting a raw that good is rare unless you do it yourself. some manga has let me increase the size 4-10x with a black and white dither and still come out smaller than the original without optimizations.
I would also suggest picking a font or a font set that reads well, I do believe that this work was already done by others but I don't know them off the top of my head.
other then this, have fresh eyes give the work a once over to see if any... clunky text could be touched up.