I've had to make these decisions too often in the last few years. My GSD had a 12 lb tumor (and she was also 12 years old) removed (on her spleen) and she survived and lived another 5 months. She did so well after the surgery (she was was cut all the way down her abdomen) that she actually didn't behave as if she felt any pain. I'm sure she did, but it wasn't bad.
I'm at the end of a battle with a nerve sheath tumor with the Chihuahua (I know - from GSD to Chihuahua??) that I rescued a few months after she passed. The only option was radiation and they gave me a low likelihood that it would help her pain (what a cruel cancer - it will not spread, it will never kill her, but damages the nerve and causes extreme pain). She had a stroke while under anesthesia during the CT scan to figure out the extent of the tumor and at that point, we gave up. Since they have to put them under anesthesia for every radiation treatment, we knew she likely wouldn't survive.
But then, I realized that we would rather die fighting than to give up. She's maybe 9 or 10 (a rescue) but a small breed with a typically long life span. So we moved forward. It was crazy expensive, I had to miss a lot of work (very understanding employer!), there was a LOT of travel (it was over an hour to the only place that did canine radiation in our area) and I held my breath every time she went under.
We have bought almost a year and I just have to figure out when she's had enough and is ready to go. Will I say I'm glad I did it? Instinctively, I say yes, because I love her so much. Realistically, if I had lost her on the day she had the stroke (and they gave her a low chance of survival that day), it may have been easier on both of us. I don't mean because of what we went through, but because I loved her more every day. So now it's that much harder. Do I regret the radiation? Maybe. The medications (she's on 7 in the morning and 5 at night) make her bloated, fat, tired, and foggy-brained. But I've never been able to let her go. Maybe it's because I just had to let one go before I met her.
Either way, I think you do what you find is right in your heart. I somehow needed to know that I reasonably did all that I could. I know now that I have. They have offered an MRI (at almost $3,000) to figure out what is causing her pain to return, but no matter what the findings, there's nothing more they can do. So, it's not unreasonable to pass on that.
I have no other advice, other than to size up every option and do the one that feels right.