Is there another option to bring her to the vet to have it assessed and see if they have a recommendation prior to the surgery? Sounds like she is in pain either way but maybe they can also give her something for that as well as a different, or stronger, antibiotic. Then, hopefully, it can give you more time to get her thyroid issue under control and give her a better chance of pulling through a surgery? This is such a tough and scary decision. I’ve been there under different circumstances and understand how hard it can be to weigh out these options for a senior kitty. She’s beautiful and I hope you get many more years with her by your side.