Max heart rate and oxygen levels after xplant

I am almost two years post heart transplant. I have been working out a lot and playing pickleball for exercise. I noticed my endurance levels have peaked and it seems the biggest limiting factor is my oxygen levels after working out. I get winded after an hour so of strenuous play—more than someone without a heart transplant.

I recently read that transplantees have a 40-70% of peak vo2 compared to non transplants in similar age group. It was attributed to denervation of heart. Seems this would have been good for me to know from the outset. Has anyone else experienced this reduced endurance (from “normal”). Anyone been able to break through this barrier through training or otherwise?