Nothing wrong with further compression, Taleman. If anything, I'd think that could only be considered a good thing.
My understanding is images and animations are now compressed as much as they can be compressed. Further compression would make image quality noticeably worse.
I do not know what compression method or methods are used now, original poster proposes using compressors like AVIF and WebP, but maybe those are already in use?
To solve this properly would require testing available image compressors and measuring which makes smallest file size without making image quality worse. This would need testing on the HH images, they are not typical photographs.
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.