[quickthrowman]

The key to making a functional automatic switch between heating and cooling is an outdoor air temp sensor. You could whip up something with a controller, a few relays, and two temp sensors, one for inside and one for outside.

A thermostat is dead simple, 24VAC and 4 wires: heat, cool, fan, power. You’d need to experiment and figure out where the point will be best to switch from heating to cooling based on the outdoor air temp, the controller would control this.

There might be commercial products available that do this already, but if they don’t have an outdoor air temp sensor, it won’t work.

FWIW I bid and run commercial building automation installations.