Ok, my advice would be the exact opposite.
It might feel like faces with a strong expression are “more advanced”. But strong expressions mean that there’s visible muscles and movements, and gesture drawing like we focus on here is made for drawing movement. So strong emotion often winds up easier because you don’t have to think so hard about what muscles are moving.
The main thing you use for initial proportions is a circle with a cross on it in basically all systems for faces. It doesn’t have to be a good circle, just a scribble. Same for the cross. The horizontal line is the eye line, the vertical one shows you where the nose should go. Get that in as a guide, then start working on what muscles seem most important for the expression. You won’t finish in 30s, but you aren’t ever supposed to. You do a bunch of bad drawings in a class. Then at the end you mark up each time slot with which one was best.
One 30m class gets you a lot of bad drawings.
6 gets you over 100 bad drawings. In a mere 3 hours of practice. It’d be really shocking if you were not better at faces and proportion after 100 drawings.
Spending the same 3 hours trying to get proportions right on one drawing will not have as big an effect because most of your time is spent on tiny details rather than the big picture of how do noses and eyes fit together.