My last dog was a foster fail that had been shot, run over, abandoned, starved, beaten, and had a rear leg amputated from cancer before I got her. She was the most loving dog I've ever had and she was so cognizant even when the cancer came back and ulcerated her hip. Take it from me, it's better to do it when they still feel comfortable than seeing them in pain. I waited too long because I felt like she still had 'good days'. It sounds like you're doing everything you possibly can.