As others have said, it’s quality not quantity of life that’s important. For many of the symptoms you’ve described, there are potential treatments available (arthritis, senile changes with the barking etc etc) but by the sounds of it you’ve discussed them with your own vet already.
My advice would be, as an obviously caring and concerned owner, you know your animal better than anyone else (vet, friends, STW members!!). You know how he usually behaves and you’re the best judge of whether he’s still enjoying life. Whilst that judgement is often clouded by emotion and guilt, deep down YOU are the best person to make that decision….and from what you’ve said in your OP, I think you’ve already decided.
As for “Letting them pass away over night”. I’d much rather they passed away in a controlled environment, with the owner present than watch an animal get worse and worse, then die alone over night…. but every case is different.
All the best whatever you decide.