Just to add my two cents that may be helpful: My wife has recently been diagnosed with ET with Jak2 mutation at the young age of 72.
Her platelets were rising for the last 5 years but it wasn't picked up until a recent blood test where the platelets were above "normal". Once the trend was discovered, the ball started rolling to find out what was going on.
Her platelets in 2021 were 225.
August of this year they were 625. That's when more testing was done and she was put on baby aspirin 2x per day (which she was on anyway from a recent total knee replacement) and 500mg of Hydroxyurea. It's almost a straight line of rising platelets from 2021 to now. So to add to your information, Becky was put on hydroxy when her levels were in the low 6 hundreds. Hope that helps.