Hi Michael,

This isn't a straight answer to the problem of extracting business logic from CSP pages - but if you haven't already, check out the isc.rest project. Its an open source release of the framework we've been using to migrate our CSP and Zen apps to REST (Angular front end). 

If nothing else, it provides a uniform, safe and easy to maintain way of creating a REST API from an existing application. 

My process has generally been to copy CSP business logic to a class method in the relevant persistent class and expose it as a REST Action there (see Defining Actions in isc-rest user guide).  Its possible to expose any method in this way, so depending on your situation, it could make sense to point an action endpoint to the class methods in CSP directly, or use a wrapper method.

This recent presentation is a great place to start: https://learning.intersystems.com/course/view.php?id=2102

git hub: https://github.com/intersystems/isc-rest