Attaching a different piezo disc was going to be my next suggestion, but since you already tried that and it’s still distorted (good job troubleshooting, by the way), then lets start looking at the circuit. As you’ve heard, FETs are all over the map when it comes to consistency in gain. If it is the case that you happen to have a very high gain FET then you can compensate for it by raising the value of R8…try around 2-4k; then check your bias to make sure it’s not too low. Good luck.