I’m sorry to hear about your situation! Must be very frustrating after all the precautions you took but I think most reefers who have been in the hobby for several years would agree that ich is present in every tank but is kept at bay somehow.
We saw an ich outbreak in our tank about a year in. The rabbitfish and one of the tangs started to show it. The first thing I did was spec a UV to halt/slow the lifecycle. If you look at the lifecycle (and I am sure you have) in detail the parasite is extremely virulent and will multiply exponentially if left unchecked.
After a few weeks running UV things started to improve. We could almost see the lifecycle of the ich as the waves of spots came and went, gradually diminishing.
Roll on a couple of years and 2 bulb replacements (I feel that is important) and I seldom see any white spots - but sometimes I see the odd one or two on my blue tang. They go away and don’t come back.
I hope you’re not against running UV because as I see it this is the most effective form of ich management. Our fish seem to have either developed resistance to it or more likely the UV filtration is suppressing it successfully.
I really hope you have success after your long struggle - I think it all depends on how far it has got in your tank at the point you add a UV. Hopefully it’s not too late!
I’m running an Aquamedic Helix 2 36w UV on a waterbox 6ft peninsula.
Good luck!
EDIT: I just (re-)read the excellent article by
@Jay Hemdal on the subject. As soon as I saw it I recalled reading it many times when this happened to us. Point number 1 is install a UV. I hope you go for it and have success!