Jquery – WordPress Website not loading proper Image URL's for background

So your main problem is that you are enqueue a JavaScript file and your browser is requesting it. Because the file ends with .js, the PHP interpreter doesn’t kick in.

You could try renaming it to .php, and that’d honestly work, however it wouldn’t be run in a "WordPress context", so functions such as get_the_ID would fail. Some people attempt to fix this by booting a mini copy of WordPress then, however in this case it would still fail because the URL of the JavaScript file doesn’t tell WordPress anything about the page holding it, so WordPress wouldn’t know the page and thus the field on it.

The better solution is probably to create a JavaScript variable that holds the result of the PHP execution, which is exactly what wp_localize_script does.

In the code below I’m enqueuing your JavaScript in mostly the same way as you were doing it before. After enqueueing, I’m then calling wp_localize_script with the result of your wp_get_attachment_url call.

add_action(
    'wp_enqueue_scripts',
    static function () {

        // The following three conditions are effectively the same as you had before in
        // a one-liner, I've just broken them out for clarity. If this ever stops working,
        // instead of "return" you can "echo 'first block';" or "echo 'second block';".
        if (!$id = get_the_ID()) {
            return;
        }

        if (!$featuredPortrait = get_post_meta($id, 'featured_portrait', true)) {
            return;
        }

        if (!$imageUrl = wp_get_attachment_url($featuredPortrait)) {
            return;
        }

        // Let's give it a more descriptive name. We'll use this both for
        // enqueuing the script and for localizing the script.
        $customScriptName = 'colindedeugd-custom-scripts';

        wp_enqueue_script(
            $customScriptName,
            get_stylesheet_directory_uri().'/js/background-customacf-loadscript.js'
        );
        
        // Although "localize" is associated with internationalization, it's also
        // used to pass data from PHP to JavaScript. This will create a global
        // JavaScript variable called "colindedeugd_custom_scripts" with a property
        // called "imageUrl" that contains the URL of the featured image.
        wp_localize_script(
            $customScriptName,
            'colindedeugd_custom_scripts',
            [
                'imageUrl' => $imageUrl,
            ]
        );
    }
);

The result of the above is that a global variable called window.colindedeugd_custom_scripts.imageUrl will be created, and that should hold your URL for you.

Assuming the first block of JavaScript is what was in background-customacf-loadscript.js (minus the script tags), you can replace it with:

    jQuery(document).ready(function ($) {
        // Searches for the Divi Blog Modules with this CSS Class where I want the background replaced
        var $module = $('.acf-blog-bg');

        // Checks if the module exists on the page
        if ($module.length) {
            // Retrieve the URL of the featured image using PHP
            var imageUrl = window.colindedeugd_custom_scripts.imageUrl;

            // Check if the image URL is not empty
            if (imageUrl !== '') {
                // Sets the background of the module to the URL of the image file
                $module.css('background-image', 'url(' + imageUrl + ')');
            }
        }
    });