Your perspective assignments look good, try adding some circles, cylinders or other rounded objects. It's difficult but a great way to push yourself.
I think you made an awesome decision in looking up Prokos videos, those have helped me so much in the past and I can't recommend them enough. Combining his videos with the Art School here will help you to understand the anatomy of the human figure.
Try to not get discouraged, learning this stuff takes some time. Your gestures are coming along really good and your second proportions assignment is better than your first. With your longer poses using a grid can be a great help. I know the video shows a 2x2 grid but I've used a 3x3 or a 4x4 as well so don't be afraid to do the same. That will also give you more areas of negative space to measure. Grids like that have been used by artists for hundreds of years don't be shy about using them.
Hopefully, that was helpful, keep up the good work