Your doctor has pretty much ruled out PV, even with the bone marrow biopsy. Hydroxyurea (HU) is a type of chemotherapy that can be very beneficial for patients with PV. However it’s not a medication you want to take if it isn’t necessary. It also comes with its own possible side effects. So maybe it would be better to take the Ayvakit your doctor recommended for the mastocytosis to rule out one suspected cause of your symptoms before moving onto HU.

If you read through the article link I gave you from Mayo for mastocytosis, the signs and symptoms of systemic mastocytosis may include:

Flushing, itching or hives
Abdominal pain, diarrhea, nausea or vomiting
Anemia or bleeding disorders
Bone and muscle pain
Enlarged liver, spleen or lymph nodes
Depression, mood changes or problems concentrating

Also, further down “systemic mastocytosis can affect your bones and bone marrow…” which may account for the hypocellularity.
There may be validity for your doctor to suspect mastocytosis based on your symptoms and test results.

But I’m also a firm believer that a person just has to follow their gut, and yours is staying PV. Whether you have PV or not, or if your itching and enlarged spleen are side effects of the mastocytosis or another condition, I’m getting the feeling that you’re not quite trusting your hematologist.

I think maybe seeking a second opinion from a larger teaching hospital such as Mayo Clinic might be a good thing for your peace of mind and getting a firm diagnosis.