I'd not worry about the KH personally.
The CO2 is quite another matter.
Plants do not prefer KH as a carbon source nor a direct pH, they do prefer CO2. pH and KH can be used to measure CO2 if..........the KH is entirely due to carbonates and if there are no other acids that will reduce the pH other than CO2.
Those are big "ifs" BTW.
I would also ignore the concept of a target pH.
Go with CO2.
That's the focus here.
Do NOT monkey with the KH or add any buffers of any sort.
Water changes and then CO2. This is where the focus is and should always be.