Hi!
Something that might help you is to give more relevance to the volume of the object you are drawing. There are several ways to achieve this; you already do one of them, which is to overlap shapes on top of one another to give the drawing the illusion of depth. There are still things you can do to get even more depth to your drawings that are really quick to do.
You could start changing how you use lines when you draw. For drawings like this one, I like to think of three things with my lines: darkness, thickness, and shape. I like to use darker, thicker lines for things that are closer to the viewer. Sometimes when I'm using a light-heavy reference, I'll also make the lines go thinner where the light hits the hardest and thicker where it is darkest. For shapes, if I can get away with making one shape be a single line, I will try my best to do so.
How I would change your drawing: There are lines that are farther away that I would intentionally make fainter/thinner, like the long line at the very top of the skull, or the teeth as they go farther back, so they look like they are indeed farther back. You could even make the lines for the eye sockets darker and thicker since it is both a dark area, and you want there to be depth compared to the inside of the skull.
Something else I would recommend is using charcoal instead of pencil for studies and larger pieces of paper, if you can. Charcoal kind of forces you to draw a lot faster, which means you can get a lot of drawings done quicker. If you have something that you are working on improving, getting a lot of drawings done is a lot more helpful than getting a few drawings that you spent a lot of time on.
Something that might help you is to give more relevance to the volume of the object you are drawing. There are several ways to achieve this; you already do one of them, which is to overlap shapes on top of one another to give the drawing the illusion of depth. There are still things you can do to get even more depth to your drawings that are really quick to do.
You could start changing how you use lines when you draw. For drawings like this one, I like to think of three things with my lines: darkness, thickness, and shape. I like to use darker, thicker lines for things that are closer to the viewer. Sometimes when I'm using a light-heavy reference, I'll also make the lines go thinner where the light hits the hardest and thicker where it is darkest. For shapes, if I can get away with making one shape be a single line, I will try my best to do so.
How I would change your drawing: There are lines that are farther away that I would intentionally make fainter/thinner, like the long line at the very top of the skull, or the teeth as they go farther back, so they look like they are indeed farther back. You could even make the lines for the eye sockets darker and thicker since it is both a dark area, and you want there to be depth compared to the inside of the skull.
Something else I would recommend is using charcoal instead of pencil for studies and larger pieces of paper, if you can. Charcoal kind of forces you to draw a lot faster, which means you can get a lot of drawings done quicker. If you have something that you are working on improving, getting a lot of drawings done is a lot more helpful than getting a few drawings that you spent a lot of time on.
Hope that helps!