Great work man !
I don’t like the Mylar concept but since you followed that design I can help you
About the crosstalk
99% of crosstalk will be away if you ditch the base and make the reflector floating by using 2-3 stripes rubber elastic cloth (see the picture of the material since I don’t know how it’s called exactly)
If you don’t want , at least place some rubber material on the bottom of the base because the vibrations of the shell go to the base .
Then use as much little as possible foam to separate the base from the reflector .Note that using less foam keeps shell vibrations passing to reflector but may cause by the time ‘sinking’ reflector to base (the foam goes compressed/flat)
About low sensitivity of the head.
The reflector must have high mass to be vibrated easily. Also glue the piezo straight to reflector plate.
Remove the craft foam. It depends if you hit the reflector on strong strikes . If you don’t then remove it.
Note : You use very fat cable on the reflector piezo. If you don’t glue the cable to the piezo it will most certainly detach soon. Better use light cables and try avoiding touching the base (for avoiding shell vibrations pass to the piezo)
Tip: The sensitivity of a piezo must never go beyond 40-50 .It certainly means there is something ‘wrong’ with the design and leads to crosstalk,retrigger , low dynamic range issues.
I am sure more people would help