Hi there,
We are facing the absolute same issue with out integration between SharePoint lists and MailChimp lists. The basic idea is there, but I assume if you are using Flow to automate processes you should be a bit more advanced and would need to include extra fields other that the defauls email address, first name, last name - which is currently the case with Microsoft Flow & MailChimp.
I have investigated with MailChimp as well, and according to them Microsoft have used a custom built API instead of using one provided by MailChimp on their website. MailChimp pointed me towards this article. Hope it helps you somehow, we haven't found a way to get arround this case and automate this process. Would appreciate if you find a way and share it here as well. Seem kinda shortsighted from Microsoft not thinking this through, hope it gets fixed soon.
Best,
George