How does this piezo input circuit work?
D1 should probably be a 3V3 or 5V zener, to protect from over-voltage as well as -ve voltage on the MCU inputs. Piezos can generate surprisingly large voltage pulses (into a high impedance) even before the drummer gets enthusiastic.

Otherwise I concur with the rest of the analysis; 1000uF is unlikely : 1 uF into 100 kilohms gives RC=0.1 seconds, any longer time constant would only add inconvenience.