Hello. : )
While I have not been tested for dysautonomia just yet, I do share some similarities with you. As you, my symptoms are typically not triggered with a change in positions (although this can happen, but typically an the end of a day), but are either present constantly, or are triggered by: eating large meals/specific foods (e.g. a large amount of orange juice), moving for too long, increased humidity, and amount/type of light I am expose to (sunlight seems to be worse than most artificial lights). These are the triggers. Some of the symptoms are: brain fog (pretty much constant), dizziness (caused mostly by eating too much fruit), fatigue (constant, and increased with exposure to the sunlight). And there are some other symptoms, such as retrograde ejaculation, that are constant.
So, it is not exactly like your case, but there is some overlap.
While antihistamines do not help me at all, they might help you, as you might be suffering from a mast cell disorder, which is common in EDS, and has an intersection with dysautonomia. For example, you might benefit from checking out this video: https://www.youtube.com/watch?v=Owtt8NO7ahU .
Hope that helps!