I'm so sorry. This must feel like you are getting the runaround. I hope you are in a position to get a second opinion.
I have to wonder if maybe the MDS was a misdiagnosis. Some of these blood cancers and disorders are tricky.
I also have to wonder why your doc feels you need both anagrelide and hydroxyurea. Dad (who also had ET) was on both meds, but only because a) his platelets were over 1,000 and b) hydroxy alone wasn't reducing them enough. He eventually was on only hydroxy.
There are other factors that affect stroke risk, such as your age, the mutation that's causing the ET, and whether you have ever had a clot before. You might ask your doctor if, given those factors, you could try going off one of the meds.
You might also ask your doc if taking a daily baby aspirin daily would help reduce stroke risk so you don't have to take so much of the other stuff.
My platelet count won't come down and stay down. On occasion, it will be 330 but jumps back up as I have blood tests every 2 weeks....have for years now. I also take a low dose aspirin. I did a bad thing recently and really needed a break from the drugs. After two weeks, I was starting to feel like myself again. It was great. Of course, my hemotologist doesn't know I did it but a nurse once told me that we have to be advocates for our own selfs sometimes. I'm 69 and don't look forward to feeling exhausted the rest of my life.