I don’t have any ideas but there must be a doc who will take your marathon sessions of semi hard-on seriously. Hard flaccid is completely different. Pressure in the back and tailbone? I don’t know how it’s connected to the rest of your symptoms but it could be an important one. Did you ever seek the help of a qualified osteopath (not a charlatan) ? Just to check if no bones are misplaced.
Since you have low arterial flow, perhaps the body is repairing the damaged arteries?