Basic question about accessing scripts in my plugin

Generally you do not want to access your PHP files directly within WordPress because you lose a lot of “juice”, meaning all WP functionality would have to be re-imported (and might break). WP is setup in a way that all requests to the CMS get routed to index.php and some mechanisms then decide what to do with the request.

From what you write, it sounds like a custom REST endpoint might serve you well.

Expanding your code to the following should already show some results:

add_action('rest_api_init', function () {
    register_rest_route('my-plugin/v1', '/my-page', [
        'methods' => [\Requests::POST],
        'callback' => 'my_awesome_func',
    ]);
});

function my_awesome_func()
{   
    if (!isset($returnVal))
        $returnVal = new stdClass();

    $returnVal->status = "success";
    $returnVal->info = "seems to work";

    return $returnVal;
}

(using short array syntax and) changed

'methods' => 'GET',

to

'methods' => [\Requests::POST],

which references the Requests’ class const (my preferred style – you can also use 'POST' directly).

Most likely you want to do something with the form data, so change your my_awesome_func() to something like this

function my_awesome_func(\WP_REST_Request $request)
{
    if ($request->get_param('main_license_number') !== 'something') {
        return new WP_Error('Invalid license number!');
    }

    return [
        'status' => 'success',
        'info' => 'hello',
    ];
}

Now there is only one problem left: everybody can call this endpoint, they don’t have to come through the form. Your request contains a fusion-form-nonce-1808 and I assume this can be used to verify that it was indeed submitted from the form – but seems very Avada/FusionForm/some third party plugin specific – which I have no knowledge of and is out of scope for this site. But you should try contacting them and ask how to verify it, something along the lines of:

add_action('rest_api_init', function () {
    register_rest_route('my-plugin/v1', '/my-page', [
        'methods' => [\Requests::POST],
        'callback' => 'my_awesome_func',
        'permission_callback' => function () {
            return magic_validate_my_nonce_please();
        },
    ]);
});

or with the fancy arrow function added in PHP 7.4:

add_action('rest_api_init', function () {
    register_rest_route('my-plugin/v1', '/my-page', [
        'methods' => [\Requests::POST],
        'callback' => 'my_awesome_func',
        'permission_callback' => fn () => magic_validate_my_nonce_please(),
    ]);
});