Just to rule out the piezo itself, since you have it already connected on one of the ATTiny PWM pins, can you get a sound from it using analogWriteAnalogWrite does not produce a sound.
it's time to check whether the buzzer polarity might be a factorIt's a non polar buzzer, which works no problem with the arduino itself.
Despite these not being the problem, I found the solution: there was an older ATTiny core that I never removed. It was overriding the new core with the tone() function. Once I removed the older core, the new one took effect and now the project works fine.
Thanks to everyone for their help!