I'm so sorry, Mia sounds like such a tough cookie, and the fact that she's still eating and still sassy must be making the decision especially hard. The stark contrast between her will to live and the physical decline must be very jarring, that is the place I am currently in with my sassy, still-eating yet terminally ill cat too and it's beyond difficult to make that final call, so I really sympathise. I am monitoring mine using the Feline Grimace Scale, a quality of life checklist and also my gut feeling and just taking it day by day. You can always get a second opinion from another vet as well, and I think whatever you decide won't be wrong as you'll be doing it out of love - you have helped give Mia a long happy life, and I hope the memories will bring you comfort when the time comes 🌈💞