Hopefully someone can suggest some diagnosis other than MG. Your symptoms do not sound like MG to me. Typically the voice gets hoarse and raspy and weak, not soprano like. I really think it may be time to move on to other possibilities. I'd hate to see something else missed. I hope you get this figured out.

Do you struggle with anxiety at all? Some of the things you describe can be anxiety related, but other symptoms usually accompany the breathing symptoms such as heart palpitations, tingling in the extremities, and feeling of impending doom along with the light headedness, shortness of breath, feeling of throat closing, and chest pain.