I do just about the same thing, mixing RO and tap water. Out of the tap my pH is about 8.2, a wee bit high for my taste. My tap / RO mix brings my pH to 7.2 and my KH to 4. I then inject CO2 and bring the pH down to 6.8.
Since you are using DIY CO2 injection, not pressurized, how do you plan on using a pH controller? You need a regulator and solenoid to run a controller.
Bumping your KH up a wee bit might not be a bad idea.