Or just active control? Set a desired output water temp, and let a computer constantly fiddle with the valves slightly to keep the output constant. Monitoring the current temp of the input lines would help but not be necessary.

I mostly don't like that every few minutes, I have to turn down the cold water a smidge, as fresher colder water from the ground comes up through the pipe. My shower has two valves for hot and cold separate, so I turn the hot water all the way on, then add just enough cold water to achieve human shower temps at full pressure.