Yikes, both courses and the workflow is still messing you up? Sorry to hear that..
I think my best advice for hardsurface sculpting is to not expect perfection. You could lose a lot of time trying to get perfect hard edges and flat surfaces, but it’s not worth the effort. In both courses I sculpted only until I could retopologize it cleanly. The retopology is where perfect hard edges and surfaces happen.
With that in mind, I really only use the flatten brush, grab brush, pinch brush, and crease brush, and smooth brush when sculpting hard surfaces. With a complex character like this, it’s the same strokes and techniques over and over. The flatted brush by itself is an imperfect brush. I often use it in conjunction with the smooth brush.
I hope some of that helps.
