I completely support you in letting this little dog go to Doggie heaven. It is very much on a Train of Diagnosis and Treatment once you get into the medical community. And once you step foot into the specialists, you are on the Expert Train of "We won't stop until you tell us to." At least, with veterinary care, you (the generic you) can slow it down and talk about cost even if that's not the real issue.
The real issue is exactly as you say: her pain and quality of life.
You are her mom. No one else is her mom. You know what is best for her.