May I suggest you START by checking each end is working as you expect?
ie (#12 @groundFungus ) if you send 64 to the ESP32 DAC do you get ABOUT 0.8V?
and - more importantly - if you connect a single cell battery - eg a fresh AAA alkaline to the Itsy Bitsy M4 analog input - you get a reading that corresponds to ABOUT 1.5 volts?