My situation is the opposite of yours, my NO3 is off the charts. Too purple to read on the Red Sea NO3 test. PO4 is zero on my Hanna meter. I’m 2.5 months into my cycle. Tanks have cycled. To do large water changes wouldn’t be easy. I have 500 gallons total in my system. I hooked up my ATS and started dosing KH2PO4. My NO3 levels are coming down and I have algae starting to grow on the ATS screen. Been watching the BRS and WWC videos. I’m going to do the 4 month cycle as per the videos. Just sent a water sample to Triton to see where things are with the water. Started with dry rock, but I also have some 3+ year old rock in my temporary tanks and some LR from KP Aquatics that I’m going to move into the new setup in the next couple of weeks.
Good luck with your setup.