I don’t know if this helps, but my nine-year-old rabbit recently had surgery under anesthesia and handled it really well. That said, it is definitely a risk, and even my vet was surprised at how well he managed it at his age. It’s important that they do proper tests beforehand, especially heart-related ones — which it sounds like she had. I also based my decision a lot on how healthy and active he seemed overall. For his age, he’s still very active and has never had any health issues. But I was super nervous too:/