I've been taking drawing seriously for only about 10 months, so take my feedback with a grain of salt!
Overall really good! I'd say for the 30 second ones, make sure to always have a clear landmark on the head to establish its orientation, like the ears you have in some of them or a line for the front of the face. For the 1 minute ones, you do already do a decent job showing overlap of the different volumes, but I think you could get a bit more if you worked on it. Particularly, the top left one is a bit flat (I also have a harder time with sitting poses). Your 5's and 10 are honestly beyond my critique. They have a great sense of volume and shape. Maybe try adding a bit more anatomical detail like muscles?
