Hey,
Just a little advice, do not start with 6000 IU vitamine D before speeking with your doc. It is not always safe. For example, it can cause kidney problems if you are sensitive to it. Only take extra vitamins you and your doc talked about.

I don't know anything about Vit D and MG, for years I take around 800 IU a day because I take prednisone. I don't know if it makes my MG worse.
My level is checked every year and stays ok.
My brother experienced some weird symptoms which turned out to be a severe vit D deficiency. He had to take a very high dosage for a few weeks, and after that he only needed 400 IU a day to keep it normal.

His symptoms were not at all comparable to my MG symptoms. Whereas his was an overall feeling of tiredness, weakness and feeling like he didn't sleep enough for weeks, mine is more pronounced,as in: you see my eye droop, my lack of facial expresssion, wobbly legs or the feeling you can't lift something.

His feeling was better described as "tired/lack energy and mine as "sick/muscle weakness".
About the cause, we don't know: he is young, healthy, gets outside a lot, eats butter (source of vit D).

Anyhow, your neurologist probably will prescribe them and maybe they'll help. If not, it's not that impossible to have both MG and a vit D deficiency, because the latter isn't that uncommon. Having vit D deficiency doesn't rule out MG.

Good luck