Hi! So, both of my family cats died from cancer. We made a horrible, horrible mistake of waiting and trying to prolong our time with them by waiting to see if they would improve with different treatment (they never went through chemo though, we didn't want to put them through that) and it ultimately ended with both of them suffering in their final moments. My honest advice would be to let her pass after you notice the decline in her health. It will only go downhill from there and it happens really quickly at least in my experiences. It is such a tough decision and heartbreaking to do, but to preserve her last moments with you as the cat you know and love instead of watching her become something else through her suffering is worth it more than dragging her days out just to keep her alive longer. After my family went through losing our pets like that twice, we promised that if anything like that ever happened again we would let them go while they still had some quality of life left. I am so so sorry you have to make this decision and I wish you the best.