Critique on long study of kneeling figure

by badgershark, March 25th 2019 © 2019 badgershark

Digital drawing. Drawn on Samsung Galaxy Tab S3 w/ stock pressure sensitive stylus in the Artflow app.

I'm looking for critique to help me improve my rendering and understanding of surface anatomy. I estimate I spent around 6-8 hours on this study, largely due to the fact that I used cross hatching, and I'm generally a slow worker who fusses around in the details a lot. I'd say I'm about 80% happy with how this study came out.

I chose to use hatching/cross hatching since it's a style I am used to and enjoy. Sometimes it helps me define the face of planes. How successful is it here?

The surface anatomy of the back can get a bit murky for me, and I feel like I'm fudging it in this study. I'm intimately familiar with anatomy since I'm in the rehabilitative health field, but the nuance of surface anatomy, where muscles lying deep under other muscles and fascia can change the landscape, can be visually confusing when I'm attempting to rend backs and shoulders.

Any tips, feedback, or help would be appreciated!