You have done everything as well as could possibly be expected with her. She's lived a long life for a captive iguana, and if she was an adult when she adopted her, she could be 20 years old. But as you know, with captive iguanas, even with perfect care they still don't get a perfect life.
I know nothing about cancer in iguanas. It's been seldom discussed here. But some people might suggest freezing as a home euthanasia method. There has been lots of discussion about that. Freezing might cause igs a lot of pain as their cells rupture when the cells' water expands, starting with the toes, tail tip and snout. No one really knows. Over the decades, I've not heard or read about vet-assisted home euthanasia for exotics. Your vet can do a painless passing for her with medicine.
Did your vet explain the details of this illness? Please repeat them in a reply. What kinds of deformed blood cells did she see? I want to think that if she's wrong about the diagnosis, then this illness could be an infection of some kind. Is it possible for you to afford a second opinion from a solidly qualified herp vet, such as at UC Davis here in California or an equal?
Again, I'm so sorry for your approaching loss, and for her hopefully slight suffering.