I'm so sorry to hear about Merc's diagnosis. We did an amp with Sweep but she also went through chemo. Before we knew it was osteo for sure, she had three rounds of doxorubicin, which is more of a broad spectrum chemo drug. That was hard on her GI wise and she needed a day or two to recover. Once we had confirmation of osteo, she was switched to carboplatin (more targeted for osteo) for the last three rounds and she never missed a beat with that. Once she had finished the IV chemo, she was put on two different oral chemo drugs, first cyclophosphamide and then later Palladia, and she tolerated both without side effects. We would have done radiation but our nearest option was Atlanta (4 hrs away) and she wouldn't have done well with all that travel. We got almost 15 months with her after the initial leg break. No signs at all before that.
For pain, gabapentin plus an NSAID plus or minus Tramadol is common. Don't let your vet be conservative. Gaba can make Merc woozy until he adjusts, so watch for that and help keep him steady if he needs support.
I can't recommend a consult with Dr. Couto highly enough. He's fantastic and will know if there's anything new in the works.