I really want to make a High Elf, and I have yet to do solstheim. My 2nd choice is Dunmer. So the decision pretty much boils down to "How much unique dunmer-PC dialogue is there".

It is hard to find out without spoilers. Could you guys who played solstheim with dunmer+other race tell me, will I really be missing out on cool dialogue if not a dunmer? Or is there only like 1-2 times your dunmer heritage is mentioned? Does it affect any quests?

I can only imagine you'd get some special dialogue or treatment if you were a Dunmer. You are, after all, in your home continent (?) I've often pondered this myself, but I'm not a fan of the Dunmer. Bosmer, if I were to be an elf. But Dunmer is definitely better than Altmer, so I say go for it.