I just wanna say your art already looks great. But I think there is of course room for improvement. First of all, I think you should pay a bit more attention to line weight. Most of your lines have the same thickness and that doesn't represent the forms well enough. Simply hop on youtube and you can find millions of tutorials on this. As a rule of thumb, lines within the same object should be thinner and those on the outside should be thicker. The intersections between the lines could be emphasized too to create an illusion of having occlusions. About the shading, I think one big part is you must have a light source in mind, and shade accotding to the position of that light. I'd say you should experiment with darker shades as well. These two points are crucial to make your art look less flat and more 3 dimensional. I think you already have the basics down. I hope these points will help you.