I lovee your style! For the expression I just pushed the side of the mouth up, and since the eyebrow is pushing down it would squish that whole area. For rendering, I used hard edges for the shadow and mid-tones, as well as airbrushed some blush on the cheeks and nose for color. You can always soften the shading in places the light hits directly. Overall using darker and brighter tones to your shading can help with that 3d look!