Frankly, your lines do actually decently indicate volume. Which makes it a bit hard to come up with the one silver bullet tip, that is guaranteed to blow you to the next level.
The big youtubers always push towards more simplification, although I must say, it drives a tear into my eyes to ignore for example Jorges neat muscle contours to achieve it. In case you need a reminder, here is a very short clip, that lists 3-D primitives for mannequinization:
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I while ago proko had this idea to completely express a pose (sans limbs) with only a box for the hip, a box for the chest and a box for the head. Sounded silly at first to me, but when I tried it out it indeed gave me some more insights about the orientation of the masses. You might want to try it, too.
As far as I have understood the idea, the way this simplification stuff "adds volume" isn't about changing so much about the lines you are currently producing when drawing from reference (apart from simplifying them and thus losing a lot of detail), it is more a preparation to go beyond the reference and allow a basis for manipulation. Like in, draw a pose simplified enough that you can reproduce the same pose, but from a different angle. And that is actually painfully hard.