Your figures look quite great, but in my opinion, you draw way too fast. The 30 sec/1 min poses aren't meant to be completed, they are there, so you spend more time on practicing the foundation for your longer drawings, and less time polishing turds.
I mean, if you ever want to progress beyond first sketches, you need something good to draw upon. If you say, want to draw 10 minutes, but you train to finish your drawing within the first minute, what are you planning to do with the 9 minutes you have left?
This is, why hands and feet are way less of a problem... you don't get a prize for sketching them in seconds, you take all the time you need to draw them properly.
Instead of speeding through the drawing process, I would recommend drawing way fewer lines in the shorties, but instead make sure, they are measured and placed properly and drawn with beautiful and controlled lines. Head and torso is by far enough for 30 secs, even for one minute, if they are designed in such a way, that you can expand them into longer pieces without constantly having to erase them or cover them in clouds of darkness to hide all the mistakes.
I mean, for how much you put down in such short time, your pieces are astonishingly good. I would recommend to really try one of them out, to see how long you can work on one of those, before you reach the point, where everything you could still add will make the piece just look worse.
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