The bird was fine for a week, but then one night I checked on him and found him sitting in his food dish, and the next morning he was dead.

I soaked all the dishes in bleach and washed then in the dishwasher, and also soaked all the wooden perches in a bleach-soap mix.
What is the best means of sterilizing the quarantine cage? Soaking it the best I can in the bathtub in a bleach-soap mix? It's 18"x30"x18".
I have never had a bird die in quarantine, nor have I ever gotten one from a pet shop. (Always from a breeder.)
This was a week ago. This weekend I was hoping to go to a bird fair and try again, as I have a lone society and a lone diamond dove that I would like companions for. But I am wary of exposure to an illness. Moreover, I only have that one room for quarantining because of my cats.
Advice on sterilizing the cage? The room? Bringing home a new bird?