I didn’t mess with the hand situation, but I’d recommend trying to use the pecs to give the chest some volume. It looks like yours are just part of a flat plane when really the rib cage is sort of egg shaped and roundish, and even then the pecs begin angling upwards along the rib cage. On the left side of the photo, the lack of volume makes the transition from chest to shoulder and collarbone look jarring to me. A few small angled contour lines might help sell that the front of the chest is not plumb with the collar bone but instead angles inward. The example isn’t super accurate, but generally that. I also noticed the back of the head sticks out. This makes the neck look a little small to me. From this angle, I think a muscular character could look better having the trapezius clearly insert into the back of the head instead of under it. The red spike material might follow that contour if you extend it into that gap between the skull and the spike.

This is really cool! I definitely second Yashwardhan and the anatomy critique. I think that strong sense of depth like you mentioned in your post can be created by staying consistent with your light source. In order to get that shadow from the hammer across his chest, your light source would have to be coming down from the top left. Losing detail in the shadows and using areas of contrast to guide the eye around the composition are useful techniques as well. You can create depth with very simple value shapes that are well organized.

The elbow isn't supposed to be that low. In order to grab the hammer the upper arm should be angled forward instead of downwards. You completely ignored how the forearm and upper arm connect because it goes out of frame. This is a basic mistake that Stan calls "no continuity".