I did a similar experiment on my blog (https://www.myersfreelance.com/chatgpt-is-good-ai-detecting-tools-are-not-and-industry-upheaval/), plugging a bunch of my articles from 2016 into AI-detectors. I found Copyleaks to be the most demanding, often scoring me in the 80s. Freelance writers, though, should strongly consider adding a provision in their contract that states they won't use AI in their work product BUT ALSO stipulating which AI-detector can be used to judge their work before action can be taken against them.