In regular auto mode the heatpump tries full climate control, so it may flip flop between heating and cooling. if you want it only constantly blowing warm air but varying fan speed them use my hack.
Here is me on a similar trip with Auto no AC. 0.1kwh for heating over 6miles, 22 degrees, 15min trip though.Yes, that was in auto, set at 22 deg. That's the point of climate control, auto and forget, mind you that was at-4 without pre conditioning. I leave the rest to Zoe.