Are you dosing NO3?
If not, then I would do water changes with the goal of keeping the nitrates under 40 ppm for sure, and under 20 would be better.
That may call for weekly 50% water changes.
I think the sporadic testing is creating more confusion than it should.
I would try this:
Plan once a week water change of 50%.
Test the tank before the water change and adjust the new water so the tank will have a set GH and KH. ie: If KH is too low in the tank, then boost the KH in the new water so high that the 50% water change will correct it. Same with GH. Write down what you did.
Test the tank mid week for GH and KH, and correct as needed. Write down what you did.
Set yourself up for success. Do not say you will test or add stuff daily, then skip it. Start by saying and doing a simpler schedule.
Go ahead and test and record the pH, too, but I would rather see the mineral levels maintain a stable level and not worry so much about the pH.
STS will remove carbonates. You will have to keep dosing for that. The oyster shell grit or coral sand will help replace some of it, but that substrate might be pretty greedy, especially at first.
1 teaspoon of baking soda in a 29 gallon tank will raise the KH by 2 German degrees of hardness.
1 teaspoon of baking soda in a 75 gallon tank will raise the KH by so close to 1 degree that I would go ahead and try that rate, then test and correct as needed. So, if you want to raise it 2 degrees in the large tank, add 2 teaspoons. You might actually need about 2.25 to 2.5 teaspoons.