Your reading in the morning before CO2 comes on is not the same as your degassed reading. And the 6.7 reading for KH of 2 doesn't make sense for degassed water. Should be closer to 7.1 or 7.2 or so.On the morning just before CO2 starting ph= 6,7 (kh = 2)
The new goal is ph drop to 5,2 for try to be sure the CO2 will not be the limit
If you think that is too much , tell me