WooCommerce by default provides the ability to disable full email notification of orders. Here you can find the settings.
Set up WooCommerce to turn off email notifications

, , , WooCommerce Autocomplete Order functions.php, "wp-content/themes/your-theme-name/:":
/**
* WooCommerce.
*/
add_action( 'woocommerce_thankyou', 'custom_woocommerce_auto_complete_order' );
function custom_woocommerce_auto_complete_order( $order_id ) {
if ( ! $order_id ) {
return;
}
$order = wc_get_order( $order_id );
$order->update_status( 'completed' );
}
,