To refine the financial program (PHP JS)

Good afternoon.
There is a program to calculate credits for several yurts. Persons.
The stack used:
— PHP 5.6
— MySQL
— Apache nginx
- docker-compose (in local development)
— Yii2 basic bootstrap
JS (basic level)

You need experience with the above stack and an understanding of design patterns.

On the task: to refine the system in accordance with the requirements of the customer.
Requirements are received weekly and it will take 4-8 hours a week to meet them.
The code is being reviewed, and time may increase.

All tasks come in the form of ready-made requirements with the development of most exceptional situations.