I had high platelets for over four years before a new GP suggested that I see a specialist. I was surprised when the specialist was at a cancer center, but a bone marrow biopsy confirmed that it was connected to a JAK 2 origin.
In the 6 months I have been on Hydroxurea my platelet count has dropped from a high of 566 to 342 and I have not noticed and negative side effects other that occasional fatigue near the end of a 4 mile hike, but I am 82 so maybe it’s concomitant with growing older. Hard to know.
I am also on low dose aspirin.

Good luck with your treatments!