A bit late to the party, but we've built a platform, Syncgrip, that will allow you to continuously sync your contacts between Airtable and Mailchimp in real-time.
Yes, it is a third-party app but it's very streamlined - will literally take you <5mins to setup.
Here's a short demo of it in action: https://youtu.be/fxegMUymY_E?t=1815 (taken from a recent appearance on the BuiltOnAir podcast to showcase Syncgrip)
Check it out here: https://www.syncgrip.com/
