It is what it is at this point with the rock my friend and that’s okay. You are over the year mark, and it can only get better.
You could try single 24 hr black outs, but the key in my opinion is trying to remove as much of the physical algae as you can to help give the bacteria a fighting chance in the space war on your rock surface.
In my past experiences with getting as much of those solids off the rocks. into the socks, swapping the socks and then doing the water change helped tremendously, though tedious and time consuming.
Theoretically some of the issues that were happening with your corals may have been nutrient based and not a black out based. But a 24 hrs period of darkness shouldn’t hurt your livestock. Dinos have a habit of becoming waterborne during the evening hours as I am sure you have read. This is also why I find it important to swap the socks before you return to your normal light cycle. You are capturing the water borne spores and removing before they reproduce in the light.
As you know, with anything that you do, the consistency that you do it with will reward you in positive results. And the age old adage of keeping it simple can never be more true. And of course patience, which can sometimes be in short supply after a long battle.
Scrub, sock, water change every 2-3 days. When less growth on the rocks, turkey baste, sock water change. And slowly it will start to shine.
BTW, 15 years in Fresh?? Would love to see a mature Fresh tank pic..
Good Luck! It’ll be worth it