In all honesty it seems you have a successful tank despite what your test kit tells you.
I had a similar experience with ammonia always testing present and no nitrates, my tank and fish are all still well after several months.
If there is not an obvious source of ammonia in the tank, like something dead in there, then I'd just keep moving forward and settle into weekly water changes, easy feeding and enjoy the tank.
I've learned not to obsess about what appears to be working