Aw, I'm so sorry to hear that :/
It sounds like you did your very best for him, and I'm sure he was very appreciative of a bigger tank.
Things like this are sometimes just down to genetics, and nothing you could have done would have saved him, it was just his time to go :/
It's a shame you're retiring, I know I was going to give up after I lost my favourite fish, but I have two rescues now who are doing great (despite all their best efforts to do away with themselves) and I'm glad I kept going!
Still, I'm sorry you lost him, and you gave him a lovely life while he was with you, and that's what's important.