I have had the nerve biopsy you speak of & wouldn't recommend it.
In my case it proved nothing.
It was several years ago but I remember like it was yesterday.
The spinal tap was a pin prick compared to it.
My foot was totally numb from local.
I was laying there with my eyes shut trying to think of something else.
I felt the tug as the surgeon made the incision but couldn't feel him probing round inside.
But when he cut through the nerve I felt a severe electric shock & saw flashes of light.
I also imagined a sound like a high tensile fence wire being cut.