one thing i find that's very helpful to look at is how animators use structure lines for their figures.
i see that you already add the center lines and eye line for the heads.
that's a very good practice, and i can definitely see progress in your head drawings.
consider possibly trying this for other body parts and limbs.
i find that always helps me whenever i'm drawing
there's probably more thorough examples, but this one shows the general idea.
hope this is helpful?