I spoke with my MPN Dr about this recently when he asked about overall symptoms. With the ratio of women to men being diagnosed with ET apparently at 2:1 and also at relatively younger ages now, hopefully this topic more broadly is looked into more. The point you make on potential common genetics is an interesting one. Hope you are doing ok Tta.

It definitely has affected mine. For me this blood flow is affected just the same as other blood flow which is constrained/doesn’t move efficiently. There is also interesting research on high platelets and high inflammation markers in endometriosis, so anyone who has that condition or related conditions might seem them being worse in the presence of ET. Tta can probably let me know from the research whether high platelets can be cause (instead of just being an effect of such a condition), but this one seems relevant: https://academic.oup.com/humrep/article/30/4/81...

Platelets are an unindicted culprit in the development of endometriosis: clinical and experimental evidence

https://academic.oup.com/humrep/article/30/4/812/615491

I'm past that, but ET never had an affect on mine.

For me, it is a very heavy period. It is regular, but very heavy. It started being a bit better after using small dose aspirin, which might seem strange, but for me it worked. Probably reducing microclotting helped reduce heavy bleeding.
As it seems I have been having higher than normal thrombocytes since adolescence, this can most likely account for the fact that my periods have always been on the heavy side.
ET can differ from person to person. Some people are more prone to clotting, some to bleeding, and some can be prone to both. And this will most likely impact how heavy periods are for someone with ET.
There are also other factors related to clotting that can impact periods too. A thrombophilia profile can help with that.

I agree that having both ET and endometriosis can be tough on a person.
From what I could find in the scientific literature, there are several parallels between endometriosis and essential thrombocythemia: both involve chronic inflammation, increased risk of thrombosis, and they both involve abnormal growth of cells.
Platelets seem to have a contribution in endometriosis, as they anyway tend to have an involvement in lesion repair, immunity, and inflammation, which is a function of the immune system. I do not know if this impact of platelets in endometriosis is more similar to their impact on solid tumour cancers (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10...), or if it is more of a hemostasis and inflammation type of impact (https://www.sciencedirect.com/science/article/p...).
I couldn't find anything showing that MPNs can increase the risk of endometriosis, most likely they just make endometriosis more severe, given that they add the two elements that would contribute to more severe endometriosis: chronic inflammation and elevated platelets.
One cannot exclude some common genetics there, between endometriosis and some MPNs, though. Hopefully science will answer these questions.
We know MPNs overlap with autoimmune disease, and autoimmune disease overlaps with endometriosis. So it stands to reason that MPNs might overlap with endometriosis too.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC28...
https://www.medrxiv.org/content/10.1101/2024.07...

We should also remember that due to medicine ignoring women's reproductive issues, endometriosis is underdiagnosed and underresearched. And as most women diagnosed with MPNs in the past were diagnosed after age 50, the interest of exploring whether these two conditions overlap was just not there.
One hopes for better science these days, both for MPNs and for endometriosis.

The Association between Endometriosis and Immunological diseases

https://www.medrxiv.org/content/10.1101/2024.07.08.24310092v1.full

The role of platelets in the regulation of tumor growth and metastasis: the mechanisms and targeted therapy

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10501337/

Autoimmunity and the risk of myeloproliferative neoplasms

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2895049/