Determining frequency of self-drive piezo oscillator circuit
This type of piezo devices are made for building sounders with minimum extra parts. They have appeared about 45 years ago when one transistor and a couple of resistors were the maximal affordable extra parts. The frequency is determined by the mechanical resonance of the piezo element, it can be forced off very little from that frequency by adding more parts, say a capacitor.

No possiblities to estimate the frequency of your circuit without a reasonably accurate math model of the piezo part, if you need something more fine than piezo manufacturer's statement 2kHz with 0,5kHz tolerance.