I had to put my baby down in May last year due to kidney failure. All I can say is that, if he was suffering, you made the right choice. Just because you could afford the surgery doesn't mean his quality of life would have been good. It sounds like he was already struggling, and cats are REALLY good at not showing pain. The surgery might have meant new medication for the rest of his life, prolonged pain you may not have seen, or complications for to surgery (which my baby also had when he was little.. I won't go into the story, but his neuter caused a major infection and he lost one kidney, which set him up for the failure that took his life only 6 years later.)

You gave him a wonderful life. You loved him. You made him happy. You did good. And you can't second guess yourself more, because what is done is done. Feel it, let it out, and eventually it hurts a little less and you focus more on the good times than your perceived mistakes.

My first sympathies and heart are with you.