Your proportions and shadows are spot on, I have nothing to comment on that! The only thing I would add is that sometimes people have sharp highlights where skin is creased and light hits it and other time it’s best not to blend all the edges. Also when drawing teeth, drawing the individual teeth give an unsettling effect! Shading the teeth to give individuality is fine but lines look uncanny. To prevent drawings from looking like they have double chins, putting a noticeable shadow under the head works nicely! I did make some changes on the hand shading too but I think you didn’t pay it too much attention because it is not the focus so I don’t have anything to say about that. Overall I think your piece didn’t need much work besides finer details, great job!
Done on adobe fresco and scratch :) NOT MY ART