My last lab was diagnosed at 8 years old. Super fit and healthy. Came up with a limp, thought it was ACL issue, turned out to be a tumor. Had her left rear leg removed by recommendation of my vet and several specialists. Went through numerous rounds of chemo afterwards. She lived another two years after diagnosis but eventually, like most osteosarcoma it spread to her lungs and progressed rapidly. We put her down just after her tenth birthday.
I don't know how I feel about it all now in hindsight. She was such an awesome dog and really part of our growing family. I didn't have any regrets then and still don't, but I just hope those last two years were worth it to her. She still grouse hunted with three legs so I'm thinking we did it right.
I hope it all works out for you. I'd get numerous opinions and options right away and see what fits best for you and your dog.