Adding a quantizer to this circuit would be overkill, but using a quantizer instead of this circuit might be a good option. After all, a quantizer module has uses besides just this.
I have a dual quantizer whose “esoteric scales” set includes all equal divisions of the octave up through 24 notes — which includes 1 note per octave, which means it puts out octave control voltages from a continuous input control voltage. There’s a knob but it just adds an offset (CV2, normalized to 5V) to the output CV which isn’t what you want for manual control but you could fake that by using both units: plug the CV into one quantizer, turn off quantization, and plug its output into the other quantizer, set up for octaves. That would just give you a manual offset of the incoming CV. Or another option would be to modify the Arduino code to add CV2 to CV1 before quantization instead of after. One limitation is that it only handles 0-5V CV, so the range is five octaves instead of the nine of the Elby circuit.