Had a similar situation with a cat, almost identical, in fact. He was giant and fluffy, we clipped him one summer to find a nub on his chest. Watched it for a year and it seemed fine, didn’t grow. Then, very suddenly, it started growing. As in your case, a matter of a week to 10 days. Fast enough that the skin started cracking, and we had to topically medicate it and clothe him in little baby shirts to keep him from gnawing at it.
I am lucky that I have a really, really honest vet. She said surgery was an option, but that it was not an option she would choose in the same situation. We took him to the hospital for a consult, and the vet there shared the same feelings. That we could attempt to remove it, but because of the location she could absolutely guarantee that the edges would not be clean. And then it would just be a matter of how long it took to get right back to the same spot…if it kept growing at that rate, it would be ~4-6 months.
Money was not an issue, quality of life absolutely was. We took him home, spoiled him rotten for a weekend, and let him go. My poor boy was only 8 and such a love, but so miserable with that thing on his chest that I couldn’t bear the thought of keeping him uncomfortable just for my sake.