In low poly modeling, you get the details mostly with Normal maps. The only parts where you get a dense topology are parts that are supposed to move (i.e. joints and face). For example I see you’ve used more polygons under the rib cage. That’s a part where normal maps are used for. You should have your topology as homogeneous as possible (relaxed).
Imo instead of tweaking with this model restart another model. I know you’ve put time and effort on it, but that’s what it takes. It takes practice to get the hang of it. But this time watch some tutorials on youtube beforehand. Also checkout this website:
It taught me a lot about face modeling and topology.