Personally if it was me, I would do bigger water changes 50-60% and dose NPK via EI to make sure there's no shortage. I didn't realize you were dosing P (didn't see it the 1st time). The dry stuff (as you mentioned) is really dirt cheap and it last forever. There's a difference relying on Nitrate from fish as opposed to dosing it. The nitrate from your tank came from Ammonia so there's more waste in your tank to get to that 20-30. It's better to dose, bigger water change, dose, water change then rely on waste.
I don't test my water anymore I just make sure I dose within EI guidelines and keep up with water changes. There's alot of wiggle room with dosing, but with waste in the tank I don't believe there is as much.