My only personal experience with these tumors was an amazingly sweet and wonderful Boxer who had been “bred to death” and then apparently thrown out when the cancer progressed. She was picked up as a stray, and I pulled her from a shelter to give her a home for whatever time she had left. In her case, she had not been treated at all, the tumors had metastasized, her lungs were affected, etc. The vet who originally examined her estimated that she had about 6 months; she ended up being with me for two very happy weeks, during which she was spoiled absolutely rotten (she gained 7 lbs, shredded a huge foam bed, tried patio dining, had toys, blankets, beds, and a buddy, etc). Sadly, the tumors were more aggressive than initially thought, and when she stopped eating and started to have more labored breathing I had her euthanized.
Don’t panic over that - Gypsy’s case was WAY WAY WAY more advanced than it sounds like your dog’s is; treatment was not remotely an option by the time she came to me. I share it only because it was an absolute worst case scenario, and I still feel like it was an incredibly positive experience for both me and her. She didn’t go downhill slowly and waste away, she was happy and eating like a pig and playing with my other dog, and over a day or two just went rapidly downhill and we knew it was time. Doesn’t mean they all will be like that, but it helped me to know that she didn’t suffer for months on end. And, when I adopted her, two friends separately told me about dogs who had received a rather dire diagnosis of mammary tumors that were beyond hope. One friend’s dog did eventually die of them, but with some creative interventions from a specialty hospital, was happy and healthy for two years post-diagnosis. The other dog ended up having a long shot surgery and chemo and recovering perfectly, and eventually dying of old age. It sounds like you are doing all the right things - hope they can get her all fixed up and healthy, she sounds like an awesome dog!