It is worth finding an MPN specialist given how rare these conditions are. Some hematologists may only see a handful of ET patients, but MPN specialists so are typically not as knowledgeable. They may be great when things are "routine" but not not be optimal when complications arise. MPN specialists see many more patients and are very focused on the range of treatment options, symptoms, etc.

I know some MPN patients see a local hematologist throughout the year, but consult with the MPN specialist annually for oversight (particularly if the specialists is not close to home). I have provided a link to one resource for such a specialist (there are other resources that provide similar lists).
https://mpninfo.org/medical-information-sources/national-international-mpn-centers/
At Dana Farber, the following doctors are listed on this source.
David P. Steensma
Martha Wadleigh