I would definitely amputate, have worked with many dogs pre and post amputation,- young and old, front and back, and they manage fine, surprisingly with a very quick transition, and if done with careful pain relief including pre and post op opioids and nerve blocks doesn't seem to be a particularly painful surgery. One way to trial is to have your vet tape the limb up so you can see how she will travel on 3 legs. Good luck with whichever decision you make.