You definitely should add a more clear outline with better blended shading. It doesn`t have to be a black outline just a darker egde to the object so you can clearly define its contours, you seem to only focus on the highlights of the image, though they can add emphasis when you are completing a drawing or something they are what you should normally be added minimally you have hardly any flat colour and seem to transition straight out of a hard highlight to a hard shading. Also your shading seems oddly inconsistent with some parts and some just are shaded really loosely, especially around the edges
I realize that most of this is due to them being sketches, but come on dude. Its on the internet. It will get judged for what it is, so please don`t use that as an excuse. Just take some time for your next thing instead of abandoning it for something you might think is better (finishing things is always cathartic) so just put an obscene amount of effort into it and even if it sucks you will probably learn something.