I can tell this was a labor of love, however I feel as though too much of your efforts are spent on the rendering part of your piece. I believe it's worth it to sit down and do musculature studies without the pressure of producing a finished/rendered piece because I can tell your muscle groupings (Notably in the chest and thighs) are guess work. Once you have a solid foundation, the rest of the rendering process will become much faster.

That aside, I believe you would benefit from locating a primary lightsource and working from there. Pillowshading can be tempting because you want to show every plane change but it works against you for the end result by making your figure look flat because its reaction to light isn't believable since there is no discernable direction the light is coming from. That also adds to your rendering workload in the end. I'm currently reading through Scott Robertson's "How To Render" book (https://e-hentai.org/g/1588812/7c6211fb4a/?p=1) to study, myself, and I think it's really worthwhile to thumb through it even though you may not necessarily be aiming for realism in your rendering.

Lastly, remember to regularly flip your canvas as you work as it helps with making sure your piece isn't leaning too far in one direction by looking at it in one way for too long.