Hilo,
I live at 8500', so, before I got my drone, I contacted Autel to make sure that the aircraft would perform okay at higher altitudes. They let me know that I should not have any problem. And, as you, (and I) now know, the aircraft do very well at higher altitudes. I also (recently) got some aftermarket props that are a tiny bit more efficient than the OEM props, and they are also a tad bit quieter. They are made by Master Screw Propellers. For $20, you might give them a try; especially since you are flying at higher altitudes.
The highest (altitude) I've had my drone so far is around 10,500, but I expect to go higher in the future. Your report of flying at higher altitudes is very encouraging for me.