I'd recommend reading
@mehaffydr's recent successful experience with hyposalinity:
https://www.reef2reef.com/threads/mehaffys-1100-gallon-plywood-display-in-north-alabama.761744/
In case it's helpful, I had ich about 18 months ago in my 120g and like you, I was so upset. I was disciplined with QT for every fish before I added the last fish that was supposed to be guaranteed QT'd, but it gave my tangs and rabbitfish ich. I set up a big QT tank, tore apart the tank to get the fish out, and treat them with therapeutic levels of copper, but the fish started to go downhill quickly. I didn't know what to do and even though they had ich, they seemed so much happier in the display so I put all the fish back in the display, plumbed my UV into the display (like you), and then fed like hell. I fed nori and all different types of frozen that was soaked in selcon, vita-chem, and a bit of spirulina powder and they all pulled through. I had to really dial up my water changes to keep the phosphates/nitrates under control with all the feeding, but it was worth it. They just naturally fought it off.
Good luck and stay determined!