I would have liked more info on the sketch to better critique it. Like duration and any (if any) pointers on what you feel your are struggling with :)
From the top of my head I'll say you have good energy and flow in your gestures. Maybe experiment with stronger lines and varied line darkness/thickness (maybe you need a softer lead for this. At least HB. Looks like you're drawing with a hard one, or you're very light on your hand ;)
30 seconds for these specific sketches. I have recently read that maybe this isn't enough time to fully absorb a pose and maybe 5-10 minutes is better. What is your opinion?
I move back and forth with hard and soft leads and at the same time feel I do the same with my line weights but don't know why.
I want to go back to basics with my fundamentals to improve my current drawing level and add more dynamic poses in my character designs.