My bullmastiff was diagnosed with lymphoma, at the age of 6, but I know the survival rates greatly differ between lymphoma and bone cancer. We treated him with chemo (6 month treatment) and he was in remission for approximately 8 months following that 6 months of treatment. When he came out of remission, we started up the chemo treatment again. He died just before his last round of chemo (adriamycin in his case), and just short of his 8th birthday. All that being said. I would 100% do it all again.
Chemotherapy in MOST dogs is not the same things that most humans experience. I honestly think the prednisone they loaded him up on affected him much more than the chemo. There is no hair loss, except in a few breeds. It certainly does not affect dogs the same way as humans. During his first 6 months of chemo, the ONLY thing we noticed was after the hardest hitting drug (the adriamycin) was that he would sleep a lot the next day. Other than that nothing was different - we still went for walks, still went on vacations with him, etc.
The second round of chemo was much harder - likely due to the fact he was a year older (7 is considered old for these big dogs) and all the drugs had been in his system before. During the second round, we also needed the anti-nausea drugs for him, when the first time he didn't lose his appetitie at all. Towards the end, his heart was the problem - we don't know if it was due to old age or the adriamycin (which can be known to weaken the heart), but the bottom line is I got almost two more years with my sweet Hutchy, and 95% of the time he was living a great quality of life.
I should also mention his lymphoma presented in his eyes, and he did lose vision in one of his eyes - but it truly did not seem to slow him down much at all. I have heard dogs can adapt to three legs very well.
I know it's a difficult decision, but I just wanted to throw in my experience. I know you will decide what is best for your sweet girl, Belle. These big dogs just get hit up so hard so young.
Here is a tribute video I made after he passed away:
ETA: The vet also told us that Hutch would only have 3 months without treatment.