Best way is still to quarantine any new additions, whether they come from a reefer of LFS. The thing is once any of your fish in the DT has ich, means that the tank is "contaminated" with the ich parasites as long as you have fishes inside.
You have a massive tank and it would be a waste not to have tangs and angels in it. You should consider catching all the fishes and then do the tank transfer method to get rid of the trophonts, then leave your DT fishless for 8 weeks to get rid of all the theronts and tomonts.