Very sorry brother. Mast cell tumors can be tough. There are things you can do that are not true chemo to be sure Luna stays as comfortable as long as possible. I am sure your vet has told you about antihistamines (mast cells release histamines). Not all MCT (mast cell tumors) are created equal. Low grades have a good survival time with nothing but removal. Same can be true for mammary tumors. It all depends on the biopsy results.
Recently put down a close friends lab for lymphoma we had discovered on a physical, after surgery and chemo kept him happy for 9 months. 2 weeks later I put my brother's lab down for bone cancer of the front leg. He elected no amputation/chemo.
Sending prayers of comfort and strength for both Luna and your family.