Its not accurate to react to low lying algae growth as a nutrient issue that requires GFO. had mentioned above about it being too harsh for many systems and how algae finds nutrients for growth even still in the best waters on earth.
low level algae growth means conditions are good for corals
GFO is for marked problems, you have merely a no grazer problem and no nutrient issues. We can link many tanks of the month right here that have higher po4 and no3 and no algae, for specific reasons. nutrient chasing to fight algae is not the best way for sure, your corals show why.
I for sure do not think your tank is crashing, you have hungry corals. The ecosystem itself shows signs of calcification, typical low level algae growth that grazers, and only grazers control in the wild, and good quality live rock not covered in any invader. coralline+
that's real ocean rock it appears and could be hundreds of years old before your tank. the reef is healthy, and the algae implies nothing other than pure balance. it does not imply a need for nutrient chasing/ although its ok to do as long as you comp feed for having done it in a tank with less than ideal mass addition to corals.
corals in an aquarium do today what we inflict in their environment 3 mos ago and sustain, if we know that time frame then we can make change X and sustain it for three mos to get a total opposite outcome now.
* Corals want to eat and take on mass when you add sw and light, and then as aquarists we begin to add things in the middle that stop that at times, its never arbitrary and always reversible we show in the threads I mailed ya
your opposites are target feeding of todays best refrigerated and frozen feeds, and water change actually to the point of sustained work, since work is guaranteed to fix any tank and nonwork is what we do when its in great condition/coral mass
you should do more than your planned water changes, so that you can feed more and dose less, its worth the work. its guaranteed to fix your tank.
Regarding the dosing, we're saying if you put your test kits away and disregard the flux, and change more than what you have scheduled your corals will be fine. I can link amazing professional tanks of the month any second taken off google that run instant ocean only, no dosing, and that's the calcium and alk ranges you report.
The level of tinkering you are doing is upper 20% tier of reefing, and plenty is done within the 80% who do not own test kits other than a swingarm hydrometer and a thermometer on the end of a wire for gauging change water.