If a WooCommerce Order has VIRTUAL checked, then simply AUTOCOMPLETE that order

automatically-complete-virtual-orders.php

/**
 * Autocomplete orders with only virtual products.
 * From the 2nd edition of Building Web Apps with WordPress
 * https://bwawwp.org
 */
function autocomplete_virtual_orders($order_id) {
	//get the existing order
	$order = new WC_Order($order_id);

	//assume we will autocomplete
	$autocomplete = true;

	//get line items
	if (count( $order->get_items() ) > 0) {
		foreach ($order->get_items() as $item) {
			if($item['type'] == 'line_item') {
				$_product = $order->get_product_from_item( $item );
				if(!$_product->is_virtual()) {
					//found a non-virtual product in the cart
					$autocomplete = false;
					break;
				}
			}
		}
	}

	//change status if needed
	if(!empty($autocomplete)) {
		$order->update_status('completed', 'Autocompleted.');
	}
}
add_filter('woocommerce_thankyou', 'autocomplete_virtual_orders');