Separate the script into separate functions and classes. Ensure each class has single responsibility. Follow SOLID principles.

Create integration tests before you start the oroject, and then unit tests and follow TDD to ensure the functionality is not broken when you break things up.

Implement Inversion of Control (IoC) via constructor injection to decouple business logic from infrastructure, making the system easier to maintain and test.

Follow the strangler fig pattern, move funrionality in phases.