You are right, some of your poses look stiff because your limbs are too short.
First thing i recommend is no matter what, do not draw from your wrist, move your entire elbow (i know its hard but try to do that as much as possible.
For limbs, make them as long as possible, let your hand flow. Dont think "I want to get the perfect length for the leg", think "I want to make the leg look flowy, as if the leg on itself has a soul"
On some of your poses it looks like you are thinking of the ribcage and pelvis as 2 seperate thing. You are thinking in 2D.
Every single movement in the human body influences a lot of muscles, if you move your arm up, your breast muscle that is connected to the deltoid will strech, the biceps will strech and so will the triceps.
Dont think of the body parts as just spheres that you have to grab in the line action, think how they are affected TOGETHER by the line of action.
Hope this helps :)
