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I can't offer much good advice or help, only some friendly words. Ziggy sounds like she's had a good life with a loving family. The way you describe her situation there's probably not much more that could be done for her even if money were no issue, but I would argue that you're already doing the most important thing: being there for her and showing her that she's loved and not alone.
Spending a pet's last days or hours together is the hardest part of having a pet. Some pets are family members, which is something pet-less people for the most part can't understand. You caring (and being strong) for Ziggy despite feeling helpless yourself is probably the most precious gift you can give her. Never forget that. Whatever happens in life that you have no control over - and cancer is a super-extra shitty example - the most important thing is to have somebody who cares about you. And for Ziggy that's you.