There is a cheaper way to keep an eye on the amount of free ammonia - use a calculator
https://www.hamzasreef.com/Contents/Calculators/FreeAmmonia.phpSet salinity to zero and enter the other figures from the tester/thermometer, and look at the "NH3 concentration" box on the right.
When there are fish in the tank, that number must be below 0.02 or the fish will suffer. Measuring free ammonia is important during fish-in cycling as we can use this to decide when we need to do a water change. But once the cycle reaches the nitrite stage, water changes should be done to remove nitrite even if free ammonia is below 0.02.
For fishless cycling, the free ammonia level doesn't matter as long as total ammonia does not exceed 5 ppm. Ideally it should not exceed 3 ppm or nitrite will go higher than stall point.
Once a tank is cycled, there should be a total ammonia reading of zero, which will always have a free ammonia level well below 0.02 regardless of pH.
Filter bacteria multiply fastest at around pH 8. They still multiply at any pH above 6.5, just not quite as fast. Adding bicarb to assist a fishless cycle in soft water has two benefits - it provides inorganic carbon for the bacteria and it increases pH. (Hard water does not need bicarb as KH is high in tap water)
But the bicarb must be removed by a total water change after the cycle finishes. Since it is only low KH water which 'needs' bicarb, GH is usually low as well, and the tank should be stocked with soft water fish which don't tolerate sodium in the water.
Baking powder is bicarbonate of soda plus an acid, usually tartaric acid. It should not be added to a fish tank.
Pick up some bicarb next time you go shopping/get a delivery.
As long as KH is over 5 that's fine. I probably used too much as my KH went to 13. This is why I suggested to use less than I did.