I have some experience with these symptoms and wanted to give my 2 cents. My daughter had POTS and dysautonomia with constipation, bloating and several other symptoms. Eventually we determined she had two parasitic infections, one was Tapeworm the other was not confirmed by pathology, but we confirmed countless microscopic samples, most likely a roundworm from pets.
You are correct in thinking the infectious disease specialist will not take this seriously. Testing is unreliable at best, and there are not many studies about pots and dysautonomia. To link these conditions to a parasite is a big leap, but I believe an accurate one.
I would encourage you to aggressively pursue the parasite theory. We used traditional RX with great success, but alt treatments may be helpful too.
The brain fog you experience is due to the byproducts like ammonia. Hepatic encephalopathy is the loss of brain function that occurs when the liver is unable to remove toxins from the blood. https://www.nlm.nih.gov/medlineplus/ency/article/000302.htm
These same byproducts from parasites can trigger dysautonomia by stimulating the adreanal glands. Do you also have an abnormal level of anxiety and or sleep problems? These are related.