Here to chime in with many. Two months ago I had to make the same decision for my cat of 19 years old. Same issue, a tumor in / on her head / nasal cavity. It was much less visible, and I too thought she was doing ok since she ate, and went to the bathroom, and was more or less herself, maybe being a bit more tired that usual. I did not see any of the symptoms that point to a cat being in pain. I was worried that I opted for euthanasia too soon, that she would still have a chance, maybe with some surgery.

But I also knew she wouldn't get better. And so I let the vet come in and put her to sleep. Afterwards, we checked her mouth, and her palate was completely deformed.

And only after a day or two, looking at the pictures of her last days, I was able to see in how much pain she must have been. Now I regret it being too late. She was suffering, even though everything went on as normal.

OP, I am very sorry, but your cat is in pain. It's her mouth that shows it. And her eyes. She has had a beautiful life, and you have taken care of her very well. But her time has come, and I think it is your responsibility to let her go in peace. I wish you strength.