I have a CM Tranquility 27, 2-ton unit. As a less expensive way of gathering some info on how much the unit spends in operation, I was thinking of adding a simple compressor cumulative runtime meter, connected to the compressor contactor. Any thoughts on that?

You can monitor that or Y1. amp draw would be could to know as well.

Very easy to do with Readington LCD hourmeters. Model 3410, around $30. I have one each on Y1 and Y2. They work fine on 24Vac - no need to make high voltage connections

I also use them to totalize run time on water heater elements - same meter works on 24 Vac or 240