So sorry for your loss.
We seriously considered amputation when Oldgirl was diagnosed back in April. Our usual vet had told us it would already have spread and is incredibly painful, so pts was the best and only option, but they could keep her comfortable for a few days for us to say our goodbyes.
The second appointment our usual vet was away, so we had to see a different, more senior vet. He questioned whether it was osteosarcoma saying it was in an atypical location and the bone pattern could be attributed to bone infection - then he tried to push us into amputation. He was similar to your vet, in that he gave us the impression it would be fairly straightforward, she would recover well and gain months of good quality, pain-free life. He actually castigated us for going for palliative, saying that if it was osteo the pain wouldn't be properly controlled no matter what we did and amputation was the only way to get rid of the pain. 
Our first vet had not doubt it was osteo and neither did we. I did a lot of research and while the position was unusual, the pattern on xray was textbook. We were also concerned that she seemed to have started coughing/spluttering. At first we weren't sure if it was just odd breathing due to the heavy painkillers, so we called our normal vet and she suggested we xray her chest.
They tried to do the xrays with her awake, but she was such a stoic bugger when it came to pain that she wouldn't lie still, so they gave her a very mild sedative. They found numerous small lesions throughout her lungs and there was no doubt they were secondaries. They also realised that somehow in less than 48 hours since she last saw the vet she had suffered a pathological fracture, so we had to let her go there and then.
You can only make choices based on the information you are given and we have to trust the experts. You did the right thing at the time, with the information you had and your boy's best interests at heart. With clear xrays and lymph nodes all the indications many of us would have made the same decision. Then, when the time came you let him go, which is the greatest act of love any of us can give our dogs.
As for feeling bad about not having time for him sometimes and occasionally being short with him, it's all part and parcel of family life, he knew he was loved and you being snappy occasionally wouldn't have changed that. No-one is able to be perfect, calm and balanced 100% of the time and we always have regrets when we lose a loved one. You didn't have anything to make up for him. You gave him a loving family home, fed him fish fingers and roast dinners and he knew how much he was loved.
Take care of yourself and give yourself time to grieve. 