It might be a good idea to add ammonia right after the water change. After each water change, the ammonia and nitrite concentration is even lower than before the water change if there was any at all to begin with. Minimize the time the bacteria lack of food source will speed up your cycle. Use both nitrate concentration and PH as indication for the need of new water change.
Your fishless cycle is obviously near the end. It should be completed in a week or so, although there is no set time. Keep it up.
Do you have a plan to add fish? Since the nitrogen cycle is near completion, the plan for what fish species, how many fish, and when to add should be considered.
