I see GH and KH also dropped when you did a water change. There is some source of minerals in the tank. I would put a small amount of everything that is in the tank or filter into a separate glass of water and test each thing separately.
Glass A = Pure tap water, just add dechlor
Glass B = A handful of the material you are using as substrate.
Glass C = A handful of the material you are using as a cap.
Glass D = A few chips of what ever rocks you have in the tank.
Glass E, F, G... = Start with just tap water and dechlor, but add however much fertilizers to each (one fertilizer per glass) that you are adding to the tank. Maybe one or more of the fertilizers is actually a blend and is adding more than you think.
...and so on until you have tested everything that you are doing to the tank, everything that is in it.
Figure out what is altering each one of those tests.
You can continue with the fishless cycle while you work it out. Water changes are not a problem, as long as you re-dose the ammonia to keep the bacteria growing.
Water changes can be good. These bacteria do not grow so well when the ammonia or nitrite are too high. Do enough water changes to keep these under 5 ppm.
How a tank smells can help you figure things out.
A 'cat pee' or 'ammonia' smell, is ammonia. While you are cycling the tank, and adding ammonia it may indeed smell like ammonia. No getting around this. After the beneficial bacteria are established, though, you should not ever smell ammonia from the tank.
A strong 'plant-swamp' smell might be from a strong odor algae or other organism. Blue Green Algae, aka Cyanobacteria is often diagnosed by this smell.
A 'Swamp gas' or 'rotten egg' smell, especially when you disturb the substrate is usually due to anaerobic bacteria. They produce hydrogen sulfide. Not good if there is so much of this you can smell it.