My tank set up is very similar to yours but about 20 % smaller. I run a calcium reactor, a skimmer most of the time, GFO until about two months ago and never change water. Total volume is about 200 gallons. Seeded the tank from the Pacific Ocean and Just kept an Octopus in it for the first year or so while it cycled.
Up until March the tank was pretty much bullet proof. Everything grew, even torn or broken 1/2 polyp frags.
I had some awesome birds nests and Stylophora. They started STN about 4 months ago after a heat wave and the tank went up by 3 degrees. I have done all manner of things to control and fix it. Eventually I just doubled the size of my Chaeto grow bin and got nutrient levels back to where they were when the tank looked its best. The last of those two species are dying now. I had a superman Monti start to die during that time but it responded well to a Iodine dip. Nothing else did. There is a thread about it some place on this forum.
During that time DKH has been from 7 to 12. Nitrate up to 40 ppm. phosphate up to .35 and back down to undetectable. Par from 600 at the highest back and down to 300. Day light from 8 to 12 hours.
The story leads up to the fact that my Acropora, montipora and Millipora have doubled and then some during that time. Previously those same corals had just sat the same size for several months.
I think it was just a bacterial infection taking out all the pocilipora. Looking back, I wish I had it in me to just pull anything that looked infected and toss it out. Seems like the rotting tissue spread from one colony to another.
IMHO the stability of my system comes from a lot of fish and nutrient being added in and a lot of Macroalgae pulling out what the coral does not need.
I had one random small year old SPS completely bleach overnight last week. But other than that I am just watching the slow demise of the birds nest and similar.
What has helped me most is turning my back on the system for a week at a time and focusing on making succulents grow in my yard. Or any other thing that I am good at. I just let whatever it was run its course. It also helps me notice the incredible growth of the acros, digis, Montis, and few LPS I have in there.
I will pull the last of the dying corals out in the next couple days, iodine dip them and put them in a new build I have going on. (I still cant make myself throw them away, some have been with me for 4 years.) Moving forward, I plan to just keep growing the things that grow well. The tank is still pretty packed even after the die off. Possibly try another Pocilipora frag when microfauna and healthy bacteria levels have had time to build back up.
I agree with one of the the previous posts. I have a reef friend that has a system running 3+ years that helps me run through things when they seem to be going wrong. It really helps me to have some one physically come over and take a look at things.
Good luck. I feel like the plan you mentioned earlier about just letting things sit and keeping fish for a while is a good one. For me, time has equaled stability. I personally dont put a ton of stock in water changes or any specific method. I do cling to some books from the early 90's and have made something that works for me.