I had the exact decision that you have to make a couple of years ago. The tumor that Ginger had grew to the size of a large baked potato almost overnight.
She seemed fine except for this growth. I took her to the vet while we debated what to do. My options were to put her down, remove tumor and test for cancer and for any spread, leave it alone and eventually it would get hooked on something and get holes and gross looking. She was healthy and happy and bouncing around. The growth was in one of the farther back areas. The vet told me that the last 2 nipples are not connected to the others so there was a good chance that it had not spread. I opted to have it removed - did not do a biopsy or any other testing as there was not much to do and knowing or not knowing really didn't matter. I should mention that my vet does laser surgery so that reduces the potential for bleeding, healing time is reduced.
3 weeks with a dog wearing a cone and keeping her quiet - just eating was a challenge for her. She however did do just fine. It only bought her another 5 months. So in retrospect should I have done it - I don't know. I know that she did get to enjoy one more summer of laying in the sun but was it worth the $ I spent - I don't know -
I do know that you have to do what your heart tells you. I chose a middle of the road option to do the removal but no further testing - it would have been easier for me if I had put her down - but she seemed to be so healthy and happy that I couldn't see doing it at the time. So assess your dogs current quality of life and do what you think is best.
Good luck whatever your decision.