Adding Segment to URL from Custom Field Value (WordPress)

I've hit a bit of a wall here. I need to take the value of a custom field ("id") and insert it into the URL of a custom post type's page (the CPT is "inventory-item"). The desired outcome is to have the URL structured as "domain.com/cars-for-sale/id/post-name." The "Cars for Sale" component and "Post Name" components are already in place per how I set up the custom post type's permalinks to work. I'm just really struggling with adding this ID segment.

I tried adding the following:

<?php

// Register custom post type
function custom_register_inventory_item_post_type() {
    // Register the custom post type
    register_post_type('inventory-item', array(
        'labels' => array(
            'name' => __('Inventory Items'),
            'singular_name' => __('Inventory Item'),
        ),
        'public' => true,
        'has_archive' => true,
        // Add other necessary arguments
    ));
}
add_action('init', 'custom_register_inventory_item_post_type');

// Add custom rewrite rules
function custom_add_rewrite_rules() {
    add_rewrite_rule('^cars-for-sale/([^/]+)/([^/]+)/?', 'index.php?inventory-item=$matches[2]', 'top');
}
add_action('init', 'custom_add_rewrite_rules');

// Handle query variables
function custom_query_vars($query_vars) {
    $query_vars[] = 'id';
    return $query_vars;
}
add_filter('query_vars', 'custom_query_vars');

// Modify the post permalink
function custom_post_type_permalink($permalink, $post, $leavename) {
    if ($post->post_type == 'inventory-item') {
        $rewritecode = array(
            '%id%',
            '%postname%',
        );
        $id = get_post_meta($post->ID, 'id', true);
        $post_slug = basename(get_permalink($post->ID));
        $rewritereplace = array(
            $id,
            $post_slug,
        );
        $permalink = str_replace($rewritecode, $rewritereplace, $permalink);
    }
    return $permalink;
}
add_filter('post_type_link', 'custom_post_type_permalink', 10, 3);

This just resulted in a 500 error so I removed the code.