Painless 3rd party marketing scripts in WordPress

functions.php

<?php

// Enqueue all js codes combined in a single file.
function enqueue_theme_assets()
{
    wp_enqueue_script( 
        'trackers', 
        get_stylesheet_directory_uri() . '/js/trackers.js', 
        null, 
        null, 
        true 
    );
}
add_action( 'wp_enqueue_scripts', 'enqueue_theme_assets' );

// Defer this script.
function add_defer_attribute( $tag, $handle )
{
    if ( is_admin() ) {
        return $tag;
    }

    // Script handles to defer
    $defer_handles = [
        'trackers',
    ];
    
    if ( in_array( $handle, $defer_handles, true ) ) {
        return str_replace( ' src', ' defer="defer" src', $tag );
    }

    return $tag;
}
add_filter( 'script_loader_tag', 'add_defer_attribute', 10, 2 );

// Preconnect all external URLs that will be requested from that scripts.
function resource_hints( $hints, $relation_type )
{
    if ($relation_type === 'preconnect') {
        $hints[] = '//www.googletagmanager.com';
        $hints[] = '//www.google-analytics.com';
        $hints[] = '//www.gstatic.com';
    }

    return $hints;
}
add_filter( 'wp_resource_hints', 'resource_hints', 10, 2 );

trackers.js

var fired = false;

window.addEventListener('scroll', () => {

    if (fired === false) {
        fired = true;

        setTimeout(() => {

            // Google Tag Manager
            (function (w, d, s, l, i) {
                w[l] = w[l] || [];
                w[l].push({
                    'gtm.start':
                        new Date().getTime(), event: 'gtm.js'
                });
                var f = d.getElementsByTagName(s)[0],
                    j = d.createElement(s), dl = l != 'dataLayer' ? '&l=' + l : '';
                j.async = true;
                j.src =
                    'https://www.googletagmanager.com/gtm.js?id=' + i + dl;
                f.parentNode.insertBefore(j, f);
            })(window, document, 'script', 'dataLayer', 'YOUR_PROPERY_ID');


        }, 1000)

    } // endif
});

The scroll event works for both desktop/traditional and mobile/touch devices, although fired a bit differently and will work most of the time.

However, if you're concerned about some users not scrolling through the page ever, you can also listen to mousemove/touchmove event. In this case I'd also adjust the timeout to a shorter value – otherwise if a user moves a mouse to some link in main menu and clicks on it navigating away from the page, your trackers won't load. But that opens up quite a rabbit hole - some users won't be captured anyway. It's up to you to decide what percentage of non-captured views you can tolerate. Do some tests.