I have always had a relatively low gas consumption. It was many years before I paid attention to how I breathed. I take a relatively slow deep inspiration, have a short pause with glottis open, have a relatively slow exhale, and then immediately inhale again. I believe this maximizes CO2 elimination, oxygen loading is not a factor. You will notice that this is the opposite way that you breathe on land. After you get control of your buoyancy, trim, and propulsion, thinking about this might assist you. I would recommend that you follow your RMV, that will give you something objective to follow as you improve all of the variable.
Good diving, Craig