First off let me say how sorry I am for you having to go through this. The “final decision” and all the questions around it are not easy to answer. From what you write your sweet Danny boy is an old dog who has reached the end of his life and the question seems to be “how” rather than “when”.
So, here my own experience. My beloved Max had reached the end of his life and I knew it for quite some time. I personally have always opted for quality of life instead of quantity. Max ate and drank well until his final day. But, he was nearly deaf, he had dementia, he broke down a few times staying unconcious for a few seconds, he was pacing around at night not finding peace, panting not finding decent sleep, caughing all the time. It had gotten to the point where I was scared to leave the house doing my groceries because he could not get up on his own once he slipped. Max was a very proud dog and I realized how hard it must have been for him to be that way.
Witnessing all those signs for some weeks I had arranged euthanasia beforehand with the vet coming to my home. Max did live another month after these arragements and the day he broke down again unable to move, I called the vet.
Max might have lived on and on like this for some more weeks but he was certainly not content anymore. And he did not move at all during “the procedure” but wagged his tail one last time when the vet talked to him. He went peacefully and within a nano-second.
In my mind, euthanasia is not murder. We have already prolonged life with all kinds of meds that some fine day just did not work anymore. My beloved Max’ time had come on that final day and it felt very right then and there to set him free.
Be sure that I did question my decision for quite some time afterwards but those were questions in favor of myself, not in favor of my beloved Max.
My heart goes out to you and your husband.