How to properly discount products in Woocommerce?

Question

WordPress

Hello.
I'm trying to make the following functionality:
apply a discount to all products of certain categories EXCEPT the first one added to the cart.
At the moment I have the following code (for the time being I am doing it for all categories, then I will add the code):

add_action('woocommerce_before_calculate_totals', 'discount', 10, 1);
function discount( $cart ) {
    if (is_admin() && !defined('DOING_AJAX')) {
        return;
    }

    if (did_action('woocommerce_before_calculate_totals') >= 2) {
        return;
    }

    $count = 0;
    $percentage = 50;

    foreach ($cart->get_cart() as $cart_item) {
        $count++;

        if ($count === 1) {
            continue;
        }

        $price = $cart_item['data']->get_price();
        $discounted_price = $price * (1 - ($percentage / 100));

        $cart_item['data']->set_price($discounted_price);
    }
}

Everything works, but I would like to somehow show the user that a discount has been applied to the products.
For example, I would like the "Total" column (subtotal product field) to display the amount including the discount.
With the above code, the "Cost" column changes and, accordingly, the "Total" column.