My cat, Emerald, was sitting next to me on July 2nd last year and I thought "her breathing is very fast" (it was 40).
On July 3rd I took her to the vet's where they discovered a pleural effusion.
On July 4th she had not improved and I took her back, for them to discover that she had lung cancer. We asked the vet to put her to sleep (or not to wake her up from the anaesthetic she had had for the X-Ray).
The vet did put her to sleep - she advised us to anyway, because Emerald was not going to get better and she had already had two anaesthetics in two days, plus drainage of the effusion. (The drainage was 300 ml, unbelievably)
So basically, I noticed her breathing was fast on July 2nd and she was dead on July 4th.
Sometimes you just cannot tell what is going to be round the corner with cats.
I think you did all you could for your cat and your love for him shines through your posts. Don't beat yourself up about it - I've read awful threads where people cannot accept that their cats are very sick, and they keep them alive at all costs. I do not think you did that. I think your cat was very loved and also very sick. You did your best for him and you were with him when he died. That's a precious gift to a much-loved friend.