I feel some of this has quite a bit of merit. It was at the time for myself, 'old tank syndrome ' then I decided to make some rather major changes years back, then 5-10 years later I made some more. It wasn't working for me in the way I envisioned it. I think this is quite a few here, at that point. I will go to bat constant for a solid CUC however. It is massive toward having a thriving system. No not sea hare, sea cucumber yes! Brittle Star? Well heck yes....sand sifting is a part of the hobby, the Conch and Cucumber do it for me, the less my hands in tank the better....Id be using nopox at half the recommend amount on a dosing pump. Especially for the phosphate high phosphate will turn coral brown and lead to tissue necrosis imo even cause brown jelly disease. Idk how cheato dies but id give it a couple attempts you can even leave the light on 24/7 its pretty simple. Id also recommend giving a deep clean to the sand bed or removing it. Also filter socks are a big help or I used to put my drain right up to the skimmer intake and it worked out decently. Really seems like old tank syndrome, and I also struggle with coral either being stagnant for years or just growing to a doom. Seems like its either one or the other. I dont think a cleanup crew is really going to do much unless you get the right animals. Big Mexican turbo snails, sand shifting conch, sea hair could be used if its alot of hair algea. And a some tangs to keep everything in check ones your algae free. You might even benefit from a rip clean and just starting over however annoying it is. Hope some of this is helpful! Happy reefing.