Am I right in thinking your vet diagnosed osteosarcoma without any xrays at all? Did you mention the previous injury to him.
I wasn't happy to pts without being absolutely sure there were no alternatives and that was at the point of being sure she had a tumour via the initial xray results - hence going for the xray under light sedation.
We had our girl for just under three weeks from initial diagnosis to when she had to be pts. She was on strong painkillers, but eating well, begging for scraps (she was a bit cheeky like that) pootling around and genuinely seemed happy. Our vet was very careful to reinforce to us how much pain she might be in without showing it, but knew how well we knew our own dog and we agreed that at the first sign she was suffering we would call a stop to things there and then. Vet also made it clear that she would only back us up as long as she was happy our girl wasn't suffering at all.
It meant we had chance to say our proper goodbyes (although no amount of time is ever enough really) and we were able to prepare our dcs for what lay ahead, which definitely helped.
It's a really hard to know what to do for best, as it's such a horrible illness and you're always aware of the 'better a day early than an hour too late' thing when you're desperate to do right by them. I would/could only say, trust your own instincts and do what feels right for you, based on your knowledge of your boy.
I have had the odd time when I've questioned whether we did the right thing keeping her with us for those weeks. But when I look at family photos of the time, I'm reassured that we did the best we could for her with the information we had at the time, backed up by a degree of gut feeling about what was right for both her and our family.