I am so sorry for your loss. I know this hurt too.
I had a 10-year-old dog that had the spleen issue as well. It hits fast, hard, without warning and is devastating. My dog's spleen had not ruptured and we did the surgery. She barely survived; many dogs do not. It is a brutal surgery with a long recovery. My dog had major infections most of the rest of her life. Please do not second guess your decision to give him a gentle passing. You gave him a final, wonderful gift and he waits for you at the Rainbow Bridge.
I know it is a huge shock to face this loss so quickly. You and your family are in my thoughts during this difficult time.
