First of all, keep in mind that I am unable to diagnose you because I am unable to examine you, this forum is for educational purposes.
MGUS causes neuropahty in 10-30% of cases, with IgM MGUS being more frequently associated with neuropathy. It is important to assess for antigen target with IgM MGUS such as Anti-MAG, ANTI-GM2 and Anti-GALOP (are all associated with neuropathies). It is true that IgM MGUS does not seem to respond to plasma exchange as well as IgG or IgA. Some centers are promoting cyclophosamide + plasma excahnge (but side effects/ possible complications are greater). Treatment for the neuropathy is generally symptomatic with neuropathic pain medications Elavil, neurontin, lyrica, tegretol, cymbalta etc. when confined to sensory symptoms only.
You rising level of M-Protein is concerning. An M-protein of 3000mg or greater would indicate a possible malignant transformation. Your bone marrow biopsy will answer this concern, and should show less than 5% plasma cells if still consistent with MGUS. I would also recommend a skeltal survey, bone scan and CT of the C-spine to evaluate for additional pathology that may explain your symptoms.
I would recommend that you see a neuromusclular specialist (neurologist that specializes in neuropathies and related disorders of peripheral nerves and muscles).
I hope this has been helpful.