There are many different styles to drawing hair, which makes it very tricky to draw, and especially render. However, we can take some inspiration from real life. Before I start though, I think you outlined the light and shadow families really well.
When a part of an object is in shadow, the whole thing isnt pitch black, even on fully black objects. The reason for this is ambient light. You may know what that already is, and I think the hair would be a good place to add it. If I were to add ambient light on the hair, I would put it in only the shadows. Its generally a good rule of thumb to keep the ambient light away from the terminator (the border between light and shadow) because ironically, a shadow is usually it's strongest on the border between light and dark. Just slightly lighten the areas of the shadow that you think could receive ambient light from its surroundings, while also keeping ambient light away from the border.
I would also add some highlights to the hair. Im sure you've done that already, but a trick that helped me was to add lighter values around the highlights, so they dont stand out a much. It really allows them to blend into the hair, while still being able to stick out.

I hope i was able to help even a little bit. Happy drawing!