So sorry for this shocking news. I applaud your difficult decision.
In case it helps others reading this post, I will share our experience. Our cat was only 3 years old when he was diagnosed with cancer in his front shoulder. We chose to amputate, and he lived ten more years.
It was a heartbreaking transition, and took our cat 2-3 months to fully adjust to life on 3 legs. As pet owners, it was difficult to watch the cat stumble and fall. Getting into the litter box with only 3 legs was a huge adjustment.
Our cat hid under our bed for six weeks, only coming out to eat or use litter box. It made us question our decision to amputate.
It is a hard decision either way. Just wanted to share our experience.