Tbh I think you'll be fine at 12AWG, the heaters will only be on full power for a limited time, and full power also decrease rapidly as the temperature increase as does the resistance, lowering the power.
You can also delay energizing the motors until the printer has been brought to temperature, and even stagger the hotend and bed heating to keep the maximum current down considerably. I'm sure you'll find ample headroom to 20A if you do some real world measurements.
But do take into account the ampacity ratings are usually given for room temperature, and might have to be derated for elevated temperatures if that is an issue.