Rheumatology can address stuff like rheumatoid arthritis, mixed connective tissue disorders, etc I've never seen one personally, but have a friend with RA who uses one, and have seen many patients who have, back when I was still practicing.

The fact that your major problem area is lower extremity weakness, suggests that you do not have MuSK MG. It is more of a "neck up" presentation. I have MuSK and even on my worst days, can walk around unassisted. That is typical for MuSK MG.

Good luck!