yah, I hear you on QT. I have had a a few lessons over the years and I can really only vouch for a few methods for a few things but everything else... LOST. I see all these people offering this service now treating with meds and such and it makes me wonder if they are just creating disease resistance themselves or if it is the volume they are dealing with and talking about that makers this apparent. A lot of fish just die on the wholesale side that we typically don't see. Its why a .30 fish to the collectors is a $100 fish to you. You can be 100% sure some are not doing everything right and just riding the message boards and experimenting with everyone else as a hobby or Buisness. I am sure some are doing it well but not sure you know who to trust.
I had a outbreak in a tank after I homed a foxface from a friend, wiped everything out except the few strong that survived and could never add a fish again. Message boards were just a mess with what I should do and I ended up just keeping my surviving fish for many years after that and never adding any again, which was fine as I started downsizing the tank with a move later. Those super fish survived a tank crashing power outage that pretty much got me out of the hobby for a long while, hence my return so many years later. Don't want to see that happen to you and yah your Fiancé needs some you time. I am sure she hates the mess in the house for what looks like months now.
Only thing I could advise is downsize your fish care to whatever you can manage in the tanks you have in the garage and let your DT fallow at a high temp so you at least know you have a safe home for your future fish vs decide you are going to let whatever is in the tank survive but limit your future goals and nix sensitive fish. Would hate to see that but it is your call.
Future QT, maybe just observe and black mollie each addition, especially with the fleet of tangs and such. If the mollie gets sick, you can abandon the fish or then try and treat. Putting all your fish through a chemical brigade preemptively is so hard to watch and we don't do that to humans. I just don't personally want to deal with endless QT on my tank this time so electing to go all captive strait from the breeders, but very limiting to fish. I chose the clowns locally because I wanted to cherry pick the patterns on them so I am doing the basic QT on them as the LFS defiantly has something in the tanks as I know they are not broken down, cleaned, etc. Clowns are probably the easiest to QT and I hate seeing them all lethargic in the tank going through the QT especially since I got them all healthy and they were likely fine. I could be responsible for their death - not saving them. I could not personally handle that with all the more sensitive wild fish and don't want to dedicate the time. Would rather focus on QT the corals etc. as that is a forever task and less stressful before they are in the tank.
One trick on adding Tangs to the tank is to use the sump as you acquire you next tang. Gets their bio load on the tank but you can then add them to the DT with your next batch of tangs. It just adds one more place to hold the fish while you sort the addition out - assuming you have room for them of corse.
Offer stands if you want to off load something just to downsize and focus on what you feel is important. I have all the basic meds and test kits with space for a smaller tank. Would treat is as a loss, but I'll do my best with them. I am sure you are way more up to speed on all of this then I with this experience so I appreciate your knowledge.
Congrats on the Wedding! Hope you can get someplace stable and have an awesome stress free honeymoon somewhere. Your future wife will be around longer then the fish hopefully, so keep her happy!