I'm probably not going to be very helpful, but I can share my experience. My first tank was a 45 gallon that I started with live rock. It was mostly LPS and softies but I was able to keep a stylo growing A LOT with zero dosing and only water changes - about a year into the tank being set up.
My current tank I started with dry rock because I didn't want anything to do with the hair algae and other pests that I had to deal with before. I did everything that I was supposed to with this one..... tested frequently, got a doser ready to accommodate alk and ca changes when the time came. I put in my first SPS 2+ yrs into it's life. Frag after frag, it would STN or RTN - sometimes in days, sometimes in weeks. I thought it was because I was chasing low nutrients or didn't have the right spectrum or right flow or something else. Well, when it hit the 3 year mark, sps stopped dying - same lights, same flow. Although my sps are not the most colorful, they are certainly growing and no more RTN/STN (knock on wood) for the last 6 months.
After analyzing these two situations, I tend to believe the bacterial diversity theory. I also believe that trying to keep a low nutrient tank puts one in a more precarious situation with stability. I still do the same things exactly the same as I did a year ago in terms of keeping parameters and housekeeping - my nutrients fluctuate because I don't try to manage it. I try not to add any chemicals that may demolish my bacteria colonies.
Without knowing more about your particular tank, I can only recommend NOT to start over and distract yourself with other LPS and Softy corals and add bacteria and phyto until you feel like your tank is stable again. I'm certain it's something we can't see or test for.
