It will keep dropping for a long time. Depends on how much baking soda I add. I was not being particularly diligent about adding baking soda to my Soil Master Select tanks, and that is still going on! (and it has been several years)
I agree, if I were not cycling the tank it would be no problem at all.
The other solution that worked in my Lake Tanganyikan tank is to blend coral sand with the substrate. Then it will not take the carbonates from the water column. I do have to prep the water with baking soda so it is right when I do a water change, but I do not have to dose more baking soda mid-week. The tank maintains the KH I set when I do a water change. I do not like the look of that, though. I like the darker substrate. Coral sand would look really bad blended with the nice color of the STS.
I might just pull one of the filters and cycle it in a bucket of water. No issue there with keeping the water parameters right. Just dose ammonia as needed.
I think the pond tablets (which were more broken up, not really tablets any more) are adding a lot of N to this tank. Nitrates are way higher than I am dosing via KNO3, and too high for the bacteria to be turning the ammonia into nitrate, yet. At about 1 week into the fishless cycle the NO2-->NO3 bacteria are just not that well developed.