I'm so, so sorry. The loss is always a rip through the heart, but for a dog that young it's worse. As you said, you really have to make the decision yourself for you and your dog. What someone else would do or could do really isn't relevant. I will say that of the people I know or know of who chose amputation for osteosarcoma, the vet's estimation of it buying maybe 6 months seems to be right.