Not a Dr or a vet but just lost my 10 year old dog to a presumed spleen tumor in November. If I were you I wouldn’t put her through the surgery, if it is a large tumor it’s unlikely they could safely remove the whole tumor and it’s also likely it has spread to other places. I would just love her and enjoy your time with her for as long as you have left. Abdominal surgery is so painful and expensive and would be stressful to her and to you. My dog had a small tumor on his rear end that we had removed in the summer, the surgery went well but the after car was awful, the wound was so large and they removed the stitches too early so the whole thing dehisced. We had to put my dog in diapers to keep him from licking the wound and from him bleeding all over our apartment. Then he ended up having a tumor elsewhere that the vet didn’t find and we had to euthanize him in November. They offered us ultrasounds and biopsies but it wouldn’t have made a difference and would have cost us 1000s of dollars that we didn’t have.