Your tank looks amazing, so sorry to hear about all this bad luck!. Regarding ich, afaik unless you leave the tank fallow for at least 6 weeks you will always have it, and it can return whenever there is heavy stress, However I have also read that if no new strains of ich are introduced, the ich continues to go thru cycles and gets weaker and weaker every time, eventualy it becomes non-infectious, I have only found one research claiming this, this period takes about 12 months. Maybe someone here can confirm this?.
This would explain why many people report having ich at the beginning and then go years without issues ever again. I have never had to deal with ich fortunately, although I have suspected ich on ocassion based on unidentified white spots, but no other symtoms, I know it is a big pain to deal with, specially with big tanks such as yours, it seems like things are going better now!, hopefully it continues that way.