@krumch I totally know about that stuff.
I meant PHP Object Interfaces.
s2-related code => Object Interface => Your core code that interacts with the WP API => Another Object Interface => the code that handles the Mailchimp stuff.
That way if you wanted to use Drip, Active Campaign, aWeber, etc. You just have to write the last part of the chain.
And if you wanted to support another Membership plugin or eCommerce solution you'd only have to change the first part.
All you have to do is write the code to implement your interfaces.
It's a Lego-style approach that I've seen a lot with payment gateway integrations.