I added baking soda while cycling a new tank with STS. It kept removing the carbonates the whole time.
So 3 weeks with lots of baking soda did not 'fill it up' with carbonates.
When I had the same problem with Soil Master Select I would add 1 teaspoon of baking soda to a 29 gallon tank and this would raise the KH by 2 German degrees and bring up the pH from 'below the test chart' to 6.2.
The fish were fine with this much change.
The math holds good no matter how you multiply or divide it.
To raise the KH by 2 degrees in a tank of 125 gallons use 4 teaspoons of baking soda. You can dissolve it in a jar of water or you can sprinkle it in dry. I would dose right into the HOB or into the path from the outlet of canister filters. But even if a blob of baking soda landed on the substrate it dissolved pretty quickly and did its job.
25% water changes are fine, if that is the correct amount as indicated by other parameters.
I would do even larger water changes, then dose baking soda mid-week between water changes.