Convert customized Wordpress Event Espresso 3x implementation to EE4x (object-oriented PHP)

We are a company offering homeschool classes, and the main function of the website is to handle class registration. We customized the Event Espresso 3 plugin - now we need to recreate the same functionality in the EE4 plugin with a custom add-on. Work has already been started, and the freelancer hired will work with our lead developer to complete discrete tasks related to the integration/transition. This small project may lead to additional development projects. See [login to view URL] for our main registration page, where users can add multiple classes to the cart, and then specify which students will be taking the classes. Our members currently create WP users and store their child/student info there. Then when logged in, all they have to do is click the check box next to the child they want to register for the class after they add it to the cart. This particular small project is to convert the default checkout process offered by the Event Espresso 4 plugin (which shows all the form fields required for every student and every class) to our customized workflow (which shows the students of the logged in member as names with checkboxes next to them, and then it pre-fills the forms and hides the ones that are complete using CSS and javascript). The desired work flow is: 1. Member logs in, and already has all of their student info stored in their account (age, bday, grade, email, etc). 2. Member browses to registration page, adds a class to cart, then clicks View Cart. 3. Member sees all of their students in a checkbox list below each class name, and can check the box next to as many students as they want to register for the class (subject to space limitations). See example cart screen grab that has two classes, and the logged in family has 4 children stored in their Family Profile. 4. A confirmation screen shows the selected classes and selected students (this is actually where the form details would be filled out in the case of a guest registrant, but for logged in members, the form is hidden), and clicks "Confirm" 5. The selections are stored in the database and members can choose a payment method. In our EE3 implementation, we overrode core plugin code, but in the EE4 version we want to make a custom add-on do the work so that it does not interfere with the ability to update the EE4 plugin. Also, EE4 was re-written with object oriented PHP and the database was restructured, so we need to make sure we can integrate our custom Wordpress account details into the new object and dB structure. We are looking to hire a developer for this part of the project, and if we are happy with their work, we have other custom plugin development and PHP code updates that need to be done, as well. The developer must be familiar with WordPress, PHP, javascript, CSS and HTML. The lead developer is working in the GMT+1 time zone, but the classes themselves and other staff are working in the GMT-4 time zone. Thanks!