Heat pumps go into defrost mode based on sum total of run times not presence of ice.
They will go into defrost even when not needed but come back out quicker based on a sensor on the coil. The “run time until defrost” is based on jumper settings on most units. I changed mine to 30 minutes since the next choice was 45 minutes based on watching then closely the first few years. It is normal for a noticeable amount of frost to be present even for several minutes before a defrost cycle.
When it is really cold, heat pump compressors actually draw less power, they are just idling along since they can’t do much due to less heat available outside.
I have outdoor temp sensors that work in conjunction with the thermostats and their firmware that can be tweaked by entering a setup mode. This is where you can program the on/off or setpoints for the electric strips. The strips always come on during defrost though or the air would be <40 deg.
Some other brands or newer designs may have defrost modes that do not keep track of run time but use sensors to decide when to defrost, this would actually be better but may be hard to keep the outside temp from affecting the sensors and causing excess defrosting.