Oh I made an experiment today and disconnected the 10kW aux electric heat strips on my 15 year old Rheem high efficiency unit and set the thermostat to 68F with an outdoor temp of 26Fhere in Leesburg, GA. I'd like to see if I can drop my electric usage this month.
I was surprised I was getting warm air coming out of the vents and it satisfied the set point after one hour of runtime just running on heat pump with electric heat disconnected.
I'm not a fan of having AC like cold air from the vents whenever the unit goes into defrost if you don't have strip heat connected to temper the air.
If you want to "automate" this for the long haul you could install an aux heat lockout (Outdoor Thermostat) kit in the heat pump. Has a rotary pot that lets you create a set point, for example 10 degrees outdoor ambient, above which the aux is always locked out. Below this it will be allowed to come on, and it will still come on during defrosts. Requires an extra wire run between the indoor and outdoor units.
Can also be accomplished with certain indoor wall thermostats that allow a remote outdoor air temp sensor to be connected and perform the same logic. Finally, some folks can't run the wires for an outdoor sensor, so some indoor thermostats have an algorithm that (when the feature is enabled) allow the pump to run for longer than normal periods of time to catch up before they would bring on aux strip heat. Helpfull when folks use a programmable thermostat and do a night time temperature set back and then the unit has to recover in the AM. I don't do set backs with a heat pump for this reason.
Hey, you may be aware of all of this....throwing it out there just in case.