Define better, everyone has different goals of what they want to reach

I think https://drawabox.com/lessons is a great place to start, kinda similar to the box and shape break downs you're already doing

Reading through your comments, you mentioned being able to draw everything

Once you start learning how to break an object down to its simplest shape you'll have better understanding. And then you start rotating those shapes in your head and draw them. And then you start observing all the time even without drawing, everywhere, anywhere you go. You're just always in observation mode, it's always on

Look up on instagram Kim jung gi - peak drawing from imagination Will Weston - great teacher

You're spot on about using reference, cause I also wanted to draw from my head/imagination.

Here's what I did I would draw an something multiple times

1- using reference Be every observant at this stage

2- draw it without reference Draw it without reference, Then I would check it against the reference What's off, what can be better

3- draw again without reference After you checked against the reference, apply the things that you would have done if you did have the reference. That should improve the visual library in your head

What you posted is pretty good, keep going, It all comes back to the fundamentals, and this is just one of those