Sonny, I'm so sorry you are going through this with your furbaby. Honestly, I don't have experience with this type of cancer in a cat, but I do have with amputations and think I can share what I would consider if I were in your shoes.
You mention that your baby had this 2 years ago and it has now recurred. I would discuss with the vet what the odds are for it returning if amputating the toe and going for radiation versus amputating the leg. The reason I would ask that is that your kitty at 12 years old "could" adjust (and they do quite readily from my experience) to a full amputation now...but if you just radiate & amputate the toe, a cat at 14+ years likely could not accept a full amputation at that age. Most vets are very leary about putting senior cats under aenesthesia, and if it recurred and you couldn't put your kitty under, then you would be faced with the decision of putting her down.
No vet will ever tell you there is "zero" chance of recurrence, so if I were in your shoes this is what I would discuss with him.
Again, my heart truly breaks for you and your kitty. Thank you for doing what you can, and we welcome you here to TCS...although I'm sorry it's under these circumstances.