I guess I'd ask the oncologist what treatment she'd need other than amputation and how long they suspect it will give her. I think there is a Facebook page dedicated to dogs with amputations. It might be Tripaws or Tripawds and you might find some more information there.
We decided against amputation for our girl who had osteosarcoma in a front leg 20 years ago. The oncologist said sure, they often do well after an amputation, he said they generally tolerate it really well, and he said she'd probably get another 4-6 good months. He said like that was a positive thing. She wasn't a lab but a larger, 120 pound dog, and we did not think that was a loving thing to do for such a minimal return. But, this was 20 years ago and veterinary medicine may have come a long way since then. I hope so. It's such a terrible disease.