Discreet way to monitor all HVAC 24VAC wires

Hello,

I'd like to monitor the 24vac wires between the thermostat and the indoor/outdoor circuitry to see what is being called for, and then be able to display those in a web page.

The reasoning is that if the thermostat is calling for fan (common to green terminal I believe) then I could also have a mass airflow sensor in the duct to tell me if the unit is actually blowing air.

Same deal with the reversing valve for the heat pump circuit, the cool circuit, the em heat (propane), etc.

Obviously I want to be very discreet and not pull a lot of amps away from the built in transformer, so perhaps even a separate sensing system like donuts might make sense, but I'm not sure how accurate they would be at that low voltage. I'd have to be watching for a couple of volts thru induction I would guess.

On the "reader" / "decoder", I would like to use a Pi on the gpio pins, but that's all going to depend on the sensors.

I cab remotely read the thermostat, but I cannot tell what the furnace/HVAC unit is doing. This is about remote viewing while not home.

The next piece after this might be to put in a high current relay that I could remotely cycle power to the furnace in case it goes down while we're away from home.

Not a fool - proof system of course, but I think it would be useful.

Thoughts?

Thanks!