You already mentioned stiff shoulders, so that's covered. I wanted to point out how stiff shoulders will cause they entire arm to ... rebel (for lack of a better word).
Your forearms are tense and raised, they need to be lowered with your elbow relaxed and easily ready to give. Your forearms can almost rest on the horse's neck to provide smooth but independent hands from the bit. (No jerking or clinging to that bit!) I disagree with having a straight line from your forearm all the way to the bit. It's not practical or correct in my eyes.
I think you also need not to turn you toes in but rather roll your thigh inward. Having the inside of your thigh resting on the saddle will correct your toes. Turn from your hip, not your ankles.
But you do look very beautiful. Your horse is lovely and picks up very well. I think you have very few flaws in this picture - well done.