Your line quality is excellent, I'm really jealous of your clean lines and I need to work to be more like you!
I've noticed your heads are anatomically a bit off pretty consistently. I think it might help to do some bone or muscle studies inside of the head again. And honestly I see some studies that you have been achieving understanding of proportion - I'm thinking it'll just take you time and you are doing all the right things.
I might have to try illustrator now, looks interesting.