Can understand your desire to keep things like shadows and highlights to a minimum low.
It is sometimes better to under do a thing. It can be too easy to push a contrast too far.
However, studying some art of the others, they can have very strong contrasts, they also have very smooth transitions.
Plus, the general rule of thumb that the smaller a figure, the greater the highlights and shadows tend to be, and the larger the figure, the opposite tend to hold true.
One fun little thing to do is to grab some fabric of a single colour (like a jacket, or a towel, a pillowcase, etc) and drape them with crinkles, wrinkles and folds. Observe how the while everything is the same colour, the tones of the colour will vary. Learning from real life!
Still, good work, and you seem to have grasped both shadows, highlights, and being subtle!
Cheers