Hi, I’m no expert but since you haven’t had any replies I thought I might try and help.
You seem to have a good understanding of shadow and perspective. My advice would be to start experimenting with lineweight to help add some dynamic to the sketch as its your job as a designer not to create a life like image but to communicate the form whilst also adding some character and personality to help sell the idea. Furthermore try to abbreviate the form with the colours and shading rather than colouring in the whole sketch. Lastly i would recommend practicing on analogue techniques more than digital.