THIS, this, this.
This is good advice. It's the best advice here. I bolded the best bits in it. I'm going to expand on those with advice I give my own students.
We do not improve by being safe.
We improve by trying new things, failing at them, trying them again, failing at them, and doing that a whole bunch until parts of those things become comfortable, and start to look okay, and then find themselves reflected in our style.
If you find you're bad at life drawing, that's a reason to do it more. If you find you're bad at drawing without guidelines, that's a reason to do that more, too. Same with exaggerated expressions, 'squished' faces, and so on.
You won't be creating comic pages or full illustrations out of practicing any of these things. That's not the point. It's practice. A singer doesn't jump up on stage to recite her scales at an audience. But she's pich-perfect because, every night in her own home, she practised her scales. It's the same with an artist. My figure drawings aren't that good. But that's not why I draw them. They're like scales. I practice them because, after a session of figure drawing, the next three days of character drawings I do in my regular style look far better.
I'm not saying 'keep practicing'. (I did read your opening post.) It's not about practising more, you're right, that's beginner stuff. Now, at your level, it's about practising smart.
Oh, one last thing. If you typically draw digitally, or draw small, go grab some charcoal and a book of A3 paper and just go to town with big sweeping gestures, and really loose, totally unplanned figures. Draw them fast. Draw them bad. It'll look terrible. Do it anyway. Churn through three pages a night. Use some figure reference if you're stuck for ideas, and translate that into toon proportions on the fly. That dynamism and energy you're looking for in your sketches will come from doing this. It works magic.
I hope some of the ideas above help you expand your visual repertoire. You will get out of the rut you're in, we all do eventually. But trying some of this stuff, and committing to sticking with it for a while, will help you come out of that rut faster and stronger! 