I think you ought to reconsider Ivabradine if your physician is offering it.
I recently saw the renowned Dr Grubb and he started me on Ivabradine. His explanation is that at a certain the increased heart rate is inefficient and offers no benefit. The baroreceptors sense they don't have enough blood, so the body naturally increases HR. In our case, however, due to blood pooling there is no blood to actually pump. The heart is working like crazy (using tons of energy/oxygen) to rapidly pump dry. If you can lower the HR without decreasing BP (which only Ivabradine can do) then the heart will rest longer between beats and fill with more blood resulting in a higher output per beat.
Im close to labeling Ivabradine a miracle drug for me. I'm using it with Midodrine and my standing HR went from 140-155 down to the 90-115 range within 48 hours, and I've had massive symptom reduction. Midodrine was ineffective solo. I'm certainly not "cured" and definitely still impaired, however I've improved at least 50%.
Beta blockers decrease BP which negates the benefit from reducing HR. It probably slows the heart but causes even worse blood pooling. Ivabradine is a genius solution for some POTS patients that respond well.