Disease
By following good fish keeping practices like you mentioned, it is possible to decrease the chances that you will have a disease outbreak in your tank. If that is really your goal for this particular tank, then perhaps one additional commitment you can make is to not add any more fish or plants into it. There is a little bit of risk anytime you do that, even if you quarantine.
I suppose it is a bit like our health. Some people do a lot of things that would cripple another person, and yet, they remain healthy. My granduncle is an example. He ate very badly. Violated just about everything we know about diet today. Smoked all his life. Had been doing so since he was a teenager back in the 1920's. And yet, he was healthy and active until the last few days of his life. He fell and never recovered from the physical trauma. My immediate family, on the other hand, are so prone to high blood pressure and cholesterol. We eat healthy, work out, don't smoke, etc. And yet, everyone, except me has had a stroke. We have to battle our cholesterol constantly.
I say keep up the good work. But don't be surprised if a disease breaks out one day. I only have three fish now. Before I had multiple tanks. Got rid of them and it came down to these three. But all of these guys have been with me for a while. I do not want to risk their health, therefore, they will not be getting any room mate. They will stay together until they die of old fish age.
I also find that the tank stays cleaner when I don't spend too much time tinkering with it. Tinkering with it tempts me to drop a pellet or two each time. I do my partial water changes, replace filtration media, feed a couple of times per week, etc.
