Yeah, Ménière’s and BPPV has been ruled out, thankfully.
The oddity is that the sets of symptoms waxes and wanes on the order of months. I had sudden hypertension (170/100, not overweight, ~155lbs and 5'9, decently healthy diet for sodium, etc) from 2016-2019, then within a couple months it completely disappeared and went back to normal (120/65). No known or significant changes in diet or lifestyle when it dropped or leading up to it.
I've kind of just "accepted" that I will have these various, "random" health issues, and am just doing my best to figure things out / mitigate them as they come along. The neck-tension-turning-into these "cervicogenic headaches" are the largest common hurdle to my life on a frequent basis. I can deal with the other noise of issues.
Anyway. Thanks again for responding!
The other thing that comes to mind is Ménière’s disease, a disease affecting the pressures of the inner ear. It's probably also considered if you've seen an ENT. A friend worked on his Ménière’s with diet (low salt, low glycemic index foods), and diuretics, but what is helping now is allergy shots. His ENT noticed that there was a seasonal component to his symptoms and sent him to an allergist. (The first allergist he went to did testing but found nothing; the next allergist found plenty of reactions and hit on an effective treatment.) Actually, his symptoms sound quite similar to yours. The Mayo clinic page on Ménière’s mentions various treatment modalities, though not allergy treatment.