You can't rely on measuring the KH and pH to determine how much CO2 is in the water. The equation that describes the relationship of those parameters only applies if there is nothing dissolved in the water that affects the pH or KH except CO2 and bicarbonates. Water in an aquarium almost always contains other substances that affect those two parameters. In fact using the KH/pH measurements to determine CO2 almost always gives you a CO2 number that is too high, usually way too high.
A pH controller will keep the dissolved CO2 approximately constant if you have very good water circulation so the tank water has about the same concentration of CO2 everywhere in the tank, and if the KH of the water never changes. Also, if the pH probe is near the location where CO2 enriched water is entering the tank, or where the CO2 is injected into the tank, the controller will act differently than if the two are widely separated. But, a lot of people do have success using a pH controller as you suggested.
The most certain way to keep a constant amount of CO2 in the water is to use the same CO2 bubble rate every day.