So you are working only where, which gonna be shown in final composition? (as per your concept).
About Sculpt, it's coming out cool, I think you should do more manual details for the natural look, right now too much alpha/stencil use on surface, manual details will help to break that. Also we should first do basic anatomy then move to minor details, the concept art has many features which sculpt doesn't have.
waiting for more updates, good luck.