I can only say that a grainy image is an unfortunate but necessary result for anyone who:
1/ produces a 1700+ pics game in less than 8 months;
2/ works alone in mounting the setting, posing the characters, posing the lighting, instead of a studio with different artists each doing one task;
3/ has 2XGTX1080ti to render each picture, instead of a render farm;
4/ uses IRay instead of 3DLight as a render engine, for more realistic renders
I could change 1/ and 4/, and release a game each 3 years, or a game with less realistic (and beautiful in my opinion) pictures.
I could, but I don't want to. And I can't afford to change 2/ and 3/.
Since the games are free I release them as they are.

I encourage those who don't like grainy pictures to help me in solving 2/ and 3/ and we'll both be very happy. Other than that there's nothing I can do.