What’s the best way to complete links on a certain WordPress Page with everything after the slug?

So, I found a solution that seems to work great and as expected:

function custom_username_links_modify_links($content)
{
    global $post;

    $landing_page_id = get_option('custom_username_links_landing_page');
    $registration_page_id = get_option('custom_username_links_registration_page');

    if ($post && $post->ID == $landing_page_id) {
        $url_path = parse_url($_SERVER['REQUEST_URI'], PHP_URL_PATH);
        $url_parts = explode("https://wordpress.stackexchange.com/", trim($url_path, "https://wordpress.stackexchange.com/"));

        if (count($url_parts) > 1) {
            $element = $url_parts[1];
            $custom_username = get_user_meta(get_current_user_id(), 'custom_username', true);
            if (!empty($custom_username) && strpos($element, $custom_username) === 0) {
                $dom = new DOMDocument();
                libxml_use_internal_errors(true);
                $dom->loadHTML($content, LIBXML_HTML_NOIMPLIED | LIBXML_HTML_NODEFDTD);

                $links = $dom->getElementsByTagName('a');
                foreach ($links as $link) {
                    $href = $link->getAttribute('href');
                    if (custom_username_links_is_safe_url($href)) {
                        $url = rtrim($href, "https://wordpress.stackexchange.com/");
                        $newHref = $url . "https://wordpress.stackexchange.com/" . $element;
                        $link->setAttribute('href', $newHref);
                    }
                }

                $content = $dom->saveHTML();
            } else {
                // Weiterleitung auf die Registrierungsseite, wenn custom_username nicht übereinstimmt
                if (!empty($registration_page_id)) {
                    wp_redirect(get_permalink($registration_page_id));
                    exit;
                }
            }
        }
    }

    return $content;
}