great practice so far but I would like to point out on the most recent head the lips are too small and the space between the lips and chin is too large as well as the ears seem to be too big but I don't know what ref you used so I could be wrong about the ear thing. The same spacing issue is also there with the distance between the nose and the mouth so maybe the nose is just too small.
for the one above your most recent one, the same distance issue between the mouth and chin, and the ears are too small in this one as well. The nose could also be extended down to make up for the changing space between the mouth and chin. I also feel like the eyes could be bigger as well. I would when you are finished with these studies next time you do them overlay your ref on top to check for what you can improve on next time, but overall everything else looks fine