Yes, manga style is actually simpler, he could even reduce color palette. And using avif (which renpy supports and it ignores file extension if not hardcoded in the scripts), somewhere around 50% file size could be achieved vs. jpg and about 2/3 vs. webp.
Btw, i got Lessions in Love from 12 GB to 4.3 GB without visual loss, but that one uses a lot of PNG.