Myasthenia gravis is quite common, so I believe any neurologist in Chonburi (of which there are a few in the public hospital, so there are likely some in the private sector too) will have at least some experience with it and will have long-term patients under their care. Unless your myasthenia gravis is very complicated, they should be competent enough to take care of you.
But then again, Chonburi is not very far from Bangkok, where many neurologists are excellent, so it comes down to your preference.