I see it’s been a while, but I want to chime in. It sounds like a challenging situation, but it’s great that you’re looking for ways to take them out and ethically minimize suffering.
Regarding your rifle setup, stability and accuracy are key. A tripod could help with stability, as could a better scope. It may be worth investing in a higher-quality scope that allows for more transparent and precise targeting. I’ve stumbled upon an article, ‘https://pocketpetcentral.com/are-marine-iguanas-dangerous/ ,’ which might also be informative for you. I hope that helps you. Good luck!