I don’t mean to be disparaging but are you sure you did the nitrate rest correctly?
My tank is at 40 PPM right now and today is water change day. 40 by most standards is excessively high. And I am changing at least 75% every 5 days. My tank is heavily stocked but I think it is still a fraction of what you have going on. if you are only changing 10-20% weekly I don’t understand how the number could be that low.