Man, I miss the life fish bring to a tank....since the issues I had with my tank and losing the majority of my fish, I have been scared crapless to add anything to the tank. I did have two chromis survive the shenanigans, but from what the fish disease experts have said, the longer I go with them in the tank post deaths, the less likely I am to have a secondary infection.
I did ask my in laws for a hanna copper tester for christmas to get more serious about qt'ing new arrivals.
I think even if you don't do a full blown medicated QT, a 1-2 week observation period in a bare-bones tank for new fish would go a long way in improving your chances for success. I knew it in theory, but the more often I do it, the more obvious it becomes that introduction of new fish is a HUGE stressor (mostly for the new fish but also the established ones). It just seems dumb and irresponsible to order a fish online, and immediately dump it into an established population. A few days at least of rest and stress-free living in an obs tank is a must IMO.
Caveat being I've only been doing this for 2 years lol