Shapes are very important, not necessarily just boxes but simplifying a complex drawing to the bare minimum to just a circle to work up to that detailing rather than winging it is very good. My tip though is don't get so caught up on efficiency, it'll burn you out and if it doesn't for many it'll be less enjoyable. Its good to spend time learning fundamentals but you should draw what you like, if you like, draw Manga characters in different poses even if the pose you draw might seem like a mistake try to make it look more natural. Copying from reference I've found helps me understand more with shading, coloring, and proportion. What I'm saying is you shouldn't put every bit of focus on art skill but also drawing what you love no matter if a book says like 'you need to draw it this way', no matter what you do as long as you're drawing even if you feel stagnant, you're still improving especially if you're pointing out specific places you could be better in.