Create auto login links on get_checkout_payment_url in Woocommerce

Im looking at this guide https://www.pirenko.com/creating-wordpress-auto-login-links/ and Im trying to adjust it to a quote mail that uses the customer-invoice.php template.

Problems:

1. how do I fetch the correct key for the order?
2. How do I update current users with a secret key so I can call it for the url?

$secret_string=wp_generate_password( 8, false );

$login_link ='http://website.com/wp-json/sand_login/prk_route?username='.$username.'&secret='.$user_info->secret_key;

I started with removing the check for secret key, just to see if i could get it to work with email-adress to start with and added a redirect to checkout. I will add the check for the security key once I can get this updated on current user meta.

Added to functions.php

add_action( 'rest_api_init', function () {
    register_rest_route(
        'sand_login',
        'prk_route',
        array(
            'methods' => 'GET',
            'callback' => 'pirenko_login_user',
        )
    );
});

function pirenko_login_user() {

    // Validate input
    if (isset($_GET['username']) && $_GET['username']!="") {

        $username = $_GET['username'];
        $user = get_user_by('email', $username );

        if ($user) {

            //Check if secret key matches
            $check_user = get_user_by( 'id', $user->ID );

                //Login info is correct - let's proceed

                // Manage login cookies
                wp_clear_auth_cookie();
                wp_set_current_user ( $user->ID );
                wp_set_auth_cookie ( $user->ID );

                // Finally redirect user
                $order = new WC_Order($order_id);               
                $payment_page = $order->get_checkout_payment_url($order->ID);
                wp_safe_redirect( $payment_page );
                exit();
            

        }
        else {
            echo "Could not find user!";
            exit();
        }
    }
    else {
        echo "Parameters are missing!";
        exit();
    }
}

The email function in customer-invoice.php looks like this:

if ($order && $order->has_status('enquiry')) {
    $email_text = str_replace('{betaallink}', '<a class="payment" href="' . esc_url($order->get_checkout_payment_url()) . '">', $email_text);
    $email_text = str_replace('{/betaallink}', '</a>', $email_text);
}

And I updated it to this, without the secret key to start with:

if ($order && $order->has_status('enquiry')) {
    $user = $order->get_user();
    $username = $user ? $user->__get('user_email') : '';
    $login_link ='http://website.com/wp-json/sand_login/prk_route?username='.$username;
    $email_text = str_replace('{betaallink}', '<a class="payment" href="'. $login_link .'">', $email_text);
    $email_text = str_replace('{/betaallink}', '</a>', $email_text);
}

This works. The user gets logged in and redirected to cart with this url with all open orders registered on the user:

https://website.com/afrekenen/order-pay/?pay_for_order=true&key

Do I go about this totally wrong?