Why don't HVAC service people want to use outdoor temperature sensors to increase efficiency?
When a heat pump is installed it should be set up to turn on the electric heat strips when it drops below the point where the heat pump is more efficient.  This is calculated on the units performance which should be specified based on the predicted (Charted) lowest anticipated temperature for your area.  If it is using the compressor it will defrost either according to a time schedule or, on more efficient models, when it senses the outdoor coil is too cold and needs to be defrosted.  When it does this, it turns off the outdoor fan and reverses the flow of refrigerant into cooling mode.  It activates the heat strips to keep the air inside running warm.  When it reaches a set temperature or after a set period of time, it will change back into heating mode.
    They showed us this in my Residential HVAC Technician Class last year. Should do everything you want it to. All in one universal replacement heat pump defrost controller with included sensors can be set up for demand or timed defrosts. https://climate.emerson.com/documents/47d01u-843-universal-heat-pump-defrost-control-specs-en-us-1569854.pdf About $81 on Amazon.