One of my work friends had to put her cat to sleep a couple months after mine; he got really skinny and it turned out he had liver cancer that was also quite widespread

She said he acted fine until the end, though, but it was just the best time to do it.
Tbh I regret not putting my own cat to sleep sooner, but I do think it was still a good time, before she got really sick and wouldn't come out from under the bed. Her breast cancer got to a point where it was pressing on her lymph nodes in her groin area and giving her pretty bad edema which made it more awkward for her to walk. But I totally understand it being painful, it was so hard seeing her that way.