I am very sorry for your loss. It is never easy to lose a feather or furry friend. It is difficult to know when to bring a bird to the vet. Animals are very good at hiding illness. You did what you could.
I too like to learn from these horrible events to see if there is anything I can do better in the future. The only thing I can suggest is to periodically weigh your birds regularly such as with a food scale to catch any major weight changes. I learned this the hard way with one budgie and now this is part of my weekly routine.
You did everything you could to love and care for him. Floyd is over the rainbow bridge now where he can fly free.