I would settle for a Raspberry Pi board that combines sensors for temperature, CO2, VOCs, and particulate load. Maybe combined with fans that can easily be put into cracked-open windows to force air flow.

There are already makers that build heat exchangers, but there are a lot of catches and other unexpected impacts.

Install a heat pump and/or geothermal.

I meant more from an air cleaning perspective not efficiency. Tesla built a very efficient auto HVAC system that pressurizes, cleans, and recirculates air through a multistage HEPA filter ("Bioweapon Defense Mode"). That kind of cleaning is usually reserved for commercial/hospital use and very expensive to implement in the residential space right now. In the last Battery Day, Musk indicated he would like to adapt the design to home use.