Hello Roger @gabyroger, Welcome to Connect. You mentioned Orthostatic HP was ruled out. From what I've read, I think Postural orthostatic tachycardia syndrome (POTS) is another condition that can affect your blood flow and cause dizziness when standing up. Here's some information I found that sounds similar to the symptoms you describe.

"Dizziness or light-headedness when standing occurs in about 20% of older people. It is more common among people with coexisting disorders, especially high blood pressure, and among residents of long-term care facilities. Many falls may result from dizziness when standing. Older people should avoid prolonged standing.

The increased incidence in older people is due to decreases in the responsiveness of the receptors that manage blood pressure plus increases in arterial wall stiffness, which make it more difficult for arteries to move more blood to increase blood pressure. Decreases in receptor responsiveness delay the normal heart and blood vessel responses to standing. Paradoxically, high blood pressure, which is more common among older people, may contribute to poor receptor sensitivity, increasing vulnerability to dizziness when standing." --- Dizziness or Light-Headedness When Standing Up: https://www.merckmanuals.com/home/heart-and-blood-vessel-disorders/symptoms-of-heart-and-blood-vessel-disorders/dizziness-or-light-headedness-when-standing-up

Postural Orthostatic Tachycardia Syndrome (POTS): https://my.clevelandclinic.org/health/diseases/16560-postural-orthostatic-tachycardia-syndrome-pots

There is also a discussion on POTS if you want to view what others have shared --- POTS - postural orthostatic tachycardia syndrome: https://connect.mayoclinic.org/discussion/pots-postural-orthostatic-tachycardia-syndrome/

Have you thought about seeking help at a major teaching hospital or health facility like Mayo Clinic?