Module development and code questions

I have created a small module (Drupal 8) to display an "Agree to Terms" link in the Drupal Commerce shopping tunnel.
This link displays the store's condition page.
I want to display the rendered store entity instead of the conditions page. How to do this ?
I created in my type of store, a display named "Terms and conditions". I want to make this display in a modal window when the user clicks on the link.

Here is my code :

I am really lost, and need some guidance.

Drupal is capable of doing something, that I want to leverage/exploit. 

But I can't figure out how to access this component in an isolated manner, everything seems to be deeply entangled, and every class I find that does what I need, requires 5 container classes and injections and dependencies...

So I'm out here asking to see if someone can help, before I need to literally read thru the entire code base to figure it out.

I'm using HOOK_form_alter to store a string for a taxonomy entity. Because I want to be graphical, I'm using the Form API radios type combined with special CSS for my customers to choose which color they want.

Relevant portion of mymodule.module

I created a small custom module to automatically flag new or updated entities.

My code doesn't work, there must be some errors. Can you help me ? Thank you

https://git.drupalcode.org/sandbox/zenimagine-3170711

I have read through the following options I found. I am not sure which one to choose, when it comes to suitability for initial import, update convenience, future module support etc. Can you please share your experience to help me make my choice:

Core 8.9.x

Use case: