To make better lines on the digital table... Well, there are a lot of things involved, but its not that hard, the best method is to place one finger at "ctrl" and the other one at "z", then you should make a fast, confident line, dont take to long making it or it will become jagged, but also dont make it too fast or you wont be able to control it very well, it will take a time to get the right feeling, if you make it wrong, hit the buttons and do it again, in simple words "just do the lines in a confident way".
About the head thing, you could study about the heads size of the body (a child would have 4 heads, a teen 5-6 heads and an adult 7-8 heads... Or something like that, i dont really remember anymore).
Like mr. Lego said, you could also focus on drawing separate parts of the body deeply before doing a full drawing, i first started drawing an eye, then the nose and mouth, then the head, body, and so on... as i am today i forgot most of the technics, but the feeling is printed on my body so its not like you gonna have a hard time if this ever happens to you, also you can always study everything again, and you wil learn new things as long as you keep trying to improve yourself.
I must say, your drawing table lineart skill is way better than my lineart skills.
Also, youre doing good, keep doing your drawings, and keep trying to improve yourself, your last drawing brought (or bought? Dunow) me a felling of nostalgia, i wish boruto was as good as naruto