Yes, these meds (Strattera/droxidopa) could aggravate the norepinephrine surge that comes with sitting or standing in hyperadrenergic POTS: https://forums.phoenixrising.me/threads/addressing-blood-pooling.88152/#post-2405716
Methylphenidate also increases dopamine and norepinephrine in both peripheral and central synapses, but I believe it is more centrally acting (CNS). I think our dysautonomia's can lead to deficiencies of a neurotransmitter in some areas and excess in others, so perhaps you have lowered norepinephrine signaling in some regions of the brain and excess peripherally....
did a nice job of summarizing the proposed role of noradrenaline in muscle tone, and, indeed, in the article from @pattismith
the way they increased the muscle tone in the rats was by injecting norepinephrine into the CNS (in this case where the trigeminal nerve leaves the brain stem). They they were able to observe that this amplified the glutamate signaling, and then ultimately the acetylcholine signaling at the neuromuscular junction of a muscle in the jaw. This ties in with methylphenidate acting more centrally and its potential tie to symptom relief.
Obviously, acetylcholine is also critical for muscle tone. The medication pyridostigmine is used for POTS and muscle weakness and may be worth considering as well if you haven't already.