I would try to reduce the number of darkness values you use for shading, and focus more on the shapes of the shadows instead.
Two values with only pitch black dark and bright white would be the most extreme variant, when all the information has to come from the edge of the shadow.
4 values, 2 values of dark and 2 values of light is probably easier to start with.
Focus on using the shape of the borders between values to model the volume, instead of trying to depict the volume with smooth gradients. You lose way more information when losing a line then when losing a gradient. If you end up with too harsh straight lines, try breaking them up into jagged lines in critical areas before just smoothing them over.
Two values with only pitch black dark and bright white would be the most extreme variant, when all the information has to come from the edge of the shadow.
4 values, 2 values of dark and 2 values of light is probably easier to start with.
Focus on using the shape of the borders between values to model the volume, instead of trying to depict the volume with smooth gradients. You lose way more information when losing a line then when losing a gradient. If you end up with too harsh straight lines, try breaking them up into jagged lines in critical areas before just smoothing them over.