Mailchimp has an amazing, well documented RESTful API. I loved exploring all what it had to offer and using it with Google Sheets and automating the whole campaign process right at the click of the button was surely fun.
Some detailed guidance on composing marketing campaigns in Mailchimp using Google Apps Script.
Source: Automating Mailchimp Campaigns using Google Apps Script — PART I
Member of Google Developers Experts Program for Google Workspace (Google Apps Script) and interested in supporting Google Workspace Devs.