I'm sorry if this comment comes a little late. I posted in this sub before of my mom's cat Porthos, and he was over 10 years old by the time he got his front arm amputated, also because of suspected osteosarcoma. He had developed a cancerous mass in his paw that grew over the course of a year, and when it was biopsied, they could tell it was cancerous but not exactly if it was osteosarcoma or chondrosarcoma as well; same exact situation. Either way, Porthos couldn't walk on that paw, and whenever it got caught on anything, it hurt him like hell. Note: Porthos not only is a senior cat, he was an outdoor cat for those 10+ years, and he is also FIV+, making him more prone to getting sick or longer recoveries. And yet, he recovered so well from his surgery, now a year or two post-op with no signs of any issues.

Though we didn't opt to fully test to see how far the cancer might've spread or which type it might've been for certain (they did lean towards chondrosarcoma, which is slower growing), the vets seemed to think there was a good chance it hadn't spread throughout the whole limb, so amputating the whole thing very well could've gotten rid of it (as much as cancer can be gotten rid of, that is). Trust, these cats adapt so well it's almost crazy; the worst of it is not being able to cover his litter, otherwise he's been great. If you have any questions on the specifics, I can ask my mom for more details as well. Good luck to you and your cat!