My furnace has an 80,000 BTU burner (equivalent to 23,000 watts), and a 1/2 horsepower blower, which would use around 600W.

So the blower uses less than 3% of the energy used by the burner. Some of the waste heat from the blower motor probably helps heat the home, so in reality it's probably less than that.

While startup current is significant (for a very short time), starting the fan up in the morning and again in the evening after work doesn't add that much to the total power drain, or even cycling it on/off a few times an hour to maintain temperature. Starting wattage on that motor is around 1800W, but only lasts for a few seconds.

This is wrong on several levels. The energy expended by a fan is proportional to the mass flow. The energy required to accelerate the rotor is negligible compared to the ongoing energy required to keep it turning and doing work. Also, I'm pretty sure that the energy use of the fan is itself negligible compared to the heat pump.

Hm - everything people are saying makes sense. I was repeating what I've been told by literally every HVAC guy I've had work on multiple houses over the last 10+ years. If the professionals are all wrong, I suppose it could just be one of those tropes that persists in an industry?

If the fan is off for at least a minute, the energy to start it should be much smaller than the energy to keep it going. If that's not true, then fix the fan design.

What fan are you talking about ?