There seems to be alot of good advice here but too much for me to read

b4 putting in my 2 cents.
1. I would remove your sand - its full of junk and surely feeding your algae issues especially with the low nutrient reading. Which is a false reading anyways your algae is taking the Phosphates before registering. You can clean it out and add it back once the algae problem is gone!
2. Add a good size UV filter. I did not have the budget for a fancy one so got a $70 jebao 36Watt one - Does the job well and I use it as needed.
3. I ordered a long coral feeder tube of
amazon (20" baster).. It doubles for blowing detritus from the sump, sand, and rock. Most recently I used it to blow of nuisance snotty algae from my rockwork (possible dinos). I have not see anymore reappear once in suspension they get zapped by the UV. I can use this and get to every nook and cranky of the tank and it works amazing.
4. If possible get a chaeto reactor, scrubber, and refugium. This will outcompete you tanks algae especially in a controlled environment. If you run it 18+ hours when the lights are off in the main display the cheato will consume much of nutrient before the lights take off..
5. If you can't blow some of the stubborn algae get a good brush and scrap the rock.. As the algae population dimensions it will slowly stop reproducing and eventually just become manageable with few clean up crew.
6. If you have too much rock in the back consider relocating to the sump. More rock = more surface area for algae and nutrients! less is more in the tank is good especially when dealing with algae. I do believe having minimal rock in the tank helps but having algae free rock in the sump also helps since sponges can harvest this rock.
Don't give up your tank is this old now you just need to find the nutrient sink, remove it and do a few waterchange and your good to go!