Forget hobby-grade air stones, lookup the pentair "Sweetwater" Fine-Pore Diffuser.I currently have a large commercial air pump sucking air from outside to a 10" CO2 canister with soda lime. The air pump is drawing air through the scrubber and goes to 4 large 2" air stones in my sump. I will get a movie of it in action tonight.
Other less critical, but potential improvements to that idea is to recirculate the air (requires covered sump and proper intake on the air pump). Also double reactors in series for the CO2 media, although this is more for efficiency I think.
Properly implemented, items 1 to 4 should be plenty to peg pH where you want it. I expect you are on the right track just maybe your setup needs some refinement.