A good way to practice proportion is to draw perfect cubes. It can help to have a real cube on hand to look at after every attempt. Most people can find a six-sided die in their home.

Practice rotating cubes in space. Tumble them around the page with the goal to preserve their apparent size/dimensions.

Practice dividing those cubes in halves, thirds, fourths, etc. First do it by eye and then check using the x subdivision method.

Do the same for cylinders - tumble and subdivide.

When you can do that much you should be able to make more progress deconstructing photographs into basic primitives.

The thing to keep in mind here is that not everything you do in drawing will train all of the skills that are involved in drawing, and so that give-and-take you identified yourself is perfectly natural. If we want to focus our studies on proportion, then yeah - we end up focusing more on two dimensional elements, focusing in on things like negative shapes, and the relationships between the different parts of the drawing as they exist on the page. If we want to focus our studies on form however, our proportions get blown out of whack because now we're purposely avoiding a lot of those 2D tools that help with proportion, in the interest of focusing on conveying the illusion of 3D form.

Another place you'll see this kind of tug-of-war is in figure drawing. Figure drawing can be focused on from two angles - structure and gesture. Structure focuses on the solidity and proportion of the body, and can as a result produce drawings that are more stiff and rigid. Gesture on the other hand focuses on the sense of movement and motion, and can produce drawings that aren't solid nor grounded, but rather feel insubstantial.

The important thing to keep in mind when it comes to studies is that your intent and what area you wish to develop matters - doing a study without a target or goal can result in that study being more muddied in its intent, and less being beneficial as a result.

Every exercise serves to develop individual skills and individual areas of understanding - what you do with those skills however is up to you. You may learn proportion in one place, form in another, figure structure here, gesture and flow there... but in the end, these are all areas of skill and understanding you will bring to bear when combining them to produce your own work - but ideally on a more instinctual basis, since those exercises will have drilled that understanding and capacity down into your subconscious, leaving your conscious brain to focus more on what it is you wish to draw.

Hi there!, I assumed that you already do research on how to get accurate proportion I'd like to link a video about this topic because I believe it'll answer the most of your doubts https://www.youtube.com/watch?v=yDIzzJgH_i8. That said, I think that the reason of why your drawings looks lil flatter it's because you aren't using some measuring methods like sized sight or blocking, for example (if you're drawing from life or using photo references). Also, assuming that you strictly drawing using line, you might play with the different line weights. This because line weights are applying in how light affects an object, making it, by the weight of line alone, acquire volume.