While your script loop is quite clever, it appears that the fullscreen script is not loading before your script.js is. Which is the problem. I would change your enqueues to this and try again.
After another glance, I believe the issue is that you have fullpage.min which is trying to be used as the ID of the script. I think that . is an invalid character, which would therefore not be enqueuing your script as it would fail on register.
Try this out, it should work. One thing to also consider is the fact that you look like you’re using asyncronous JS loading. This means your script.js file, which is probably smaller than the assets it requires, is loading in first, meaning that fullpage is not ready yet. Try not using async, and instead enqueueing in the footer.
wp_register_script( 'easing', get_stylesheet_directory_uri() . '/assets/js/easing.js', array('jquery'), '', true);
wp_register_script( 'slimscroll', get_stylesheet_directory_uri() . '/assets/js/slimscroll.min.js', array('jquery'), '', true);
wp_register_script( 'fullpage', get_stylesheet_directory_uri() . '/assets/js/fullpage.min.js', array('jquery'), '', true);
wp_register_script( 'my-script', get_stylesheet_directory_uri() . '/assets/js/script.js', array( 'jquery', 'easing', 'slimscroll', 'fullpage' ), '', true);
wp_enqueue_script( 'my-script' );