in general, heat is dissipated faster with a larger heatsink, you could try that and see if it helps.
as for the khadas tone1 to only use the data lines and the plus/minus for the power supply will not work i already tried this once.
somehow the raspberry pi needs voltage via usb otherwise you will get an alsa error in volumio.
in itself it does not matter what according to the documentation the toneboard1 automatically switches to external voltage.
you could try it with a resistor in between, so that pi has enough voltage not to get an error.