To the OP, given the time, money and possible outlook for your dog, end of the day I'm certain both you and your wife will know what is in the dogs best interest.
We just had to take that last dreaded trip to the vet with one of my FIL's dogs. They adopted her and her sister from us about 12 years ago from a litter from a rescue we took in. FIL died last year, MIL with her allergies and COPD couldn't handle two dogs by herself (side note, same two dogs I drove down from Vermont for my inlaws when they were moving to NC and when I met Roy from this website). Sorry for the rambling, but she and her sister are like our own dogs, and it was hard for my wife due to her dad.
Since living here 18 years now, we've had to take at least 9 "last trip to vet" visits. Each trip NEVER gets easier.
Every dog of ours was different with it's age and issues, but at the end of the day, both my wife and I knew when it was time. Way I see it, if you really love your dog, you don't want them to suffer, and you don't want to put them through a lot of crap that "may or may not work" given the odds by the vet. Out of those 9 trips, I will say that none of our dogs went through a long drawn out medical procedures to prolong it's life just because we don't want to lose it.
Generally every dog making that last trip gets a steak meal and a "one on walk" outside down to the creek to enjoy the day if it's able to. Only suggestion is to be there for your animal when it's time, holding it and giving them words of praise for making such an impact in your life. I don't think a dog could ask for anything more.
We're down to 5 dogs now, and wife swears no more. I personally think we'll always have a 4 legged companion.