Introduce service classes (business logic out of controllers)

Reduce Yii:: static calls

Add PSR-3 logging (Monolog)

Improve test coverage

Isolate domain logic

This gives you 80% of the benefits with 20% of the cost.

Keep Yii2 for backend logic and API endpoints.

Build React components for views (e.g., order table, order form).

Communicate via JSON endpoints (REST API or Yii2 controller actions returning JSON).

Gradually replace old jQuery interactions.