Hey good work, you certainly seem to be putting in the effort. I am kind of interested in your formal art school education, what did you think and how it compares to what Marc has put out thus far?
As for feedback I suggest practicing fluid line work, try to get the same result with less lines, less scratchy. Gesture drawing really is the king of all exercises when it comes to this imo.
Something that also helped me especially with longer poses is Russian academic drawings. These style of drawings often begin with blocking out the figure with only straight lines, finding the angles with as few lines as possible.
If in the long term you want to have a sketchy style which I definitely also like I believe it still pays off to have the understanding and control that comes from clean lines first. Hope this helps.