App with details on Environmental Control system?

How much information you could get with such an app would depend on what the car has to give. From Gen 2 on you should be able to see the compressor rpm, and maybe the compressor watts (you can in Gen 3). Gen 3 has a pressure transducer on the high side, so you can read high-side pressure that way. Gen 2 only has a (too low)/(not too low or too high)/(too high) pressure switch. I don't know of any generation with a transducer on the low side, so you'd still need a gauge for that.

In any generation, you should be able to see the outside air temp, cabin air temp, and evaporator temp. Gen 2 had a cabin humidity sensor but Gen 3 dropped it. I think Gen 4 has one again, combined with the windshield rain sensor (or maybe a standalone humidity sensor in that location, in trims without the rain sensor).