How can I list ordered items in the custom-made confirmation letter?

Question

I use a simple custom module to send confirmation emails in Drupal 9 Commerce 2 for a custom order states.

I am basing my module off of the example shown in the handbook. The main module script resides in modules/custom/my_newmodule/src/EventSubscriber, its name is OrderProcessSubscriber.php and its code is as follows:

<?php

namespace Drupal\my_newmodule\EventSubscriber;

use Drupal\state_machine\Event\WorkflowTransitionEvent;
use Drupal\Core\Language\LanguageManagerInterface;
use Drupal\Core\Mail\MailManagerInterface;
use Drupal\Core\StringTranslation\StringTranslationTrait;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;

/**
    * Sends an email when the order transitions to Fulfillment.
    */
class OrderProcessSubscriber implements EventSubscriberInterface {

    use StringTranslationTrait;

    /**
    * The language manager.
    *
    * @var \Drupal\Core\Language\LanguageManagerInterface
    */
    protected $languageManager;

    /**
    * The mail manager.
    *
    * @var \Drupal\Core\Mail\MailManagerInterface
    */
    protected $mailManager;

    /**
    * Constructs a new OrderFulfillmentSubscriber object.
    *
    * @param \Drupal\Core\Language\LanguageManagerInterface $language_manager
    *   The language manager.
    * @param \Drupal\Core\Mail\MailManagerInterface $mail_manager
    *   The mail manager.
    */
    public function __construct(
    LanguageManagerInterface $language_manager,
    MailManagerInterface $mail_manager
    ) {
    $this->languageManager = $language_manager;
    $this->mailManager = $mail_manager;
    }

    /**
    * {@inheritdoc}
    */
    public static function getSubscribedEvents() {
    $events = [
        'commerce_order.process.post_transition' => ['sendEmail', -100],
    ];
    return $events;
    }




    /**
    * Sends the email.
    *
    * @param \Drupal\state_machine\Event\WorkflowTransitionEvent $event
    *   The transition event.
    */
    public function sendEmail(WorkflowTransitionEvent $event) {
    // Create the email.
    $order = $event->getEntity();
    $to = $order->getEmail();
    $myTime = $order->getPlacedTime();
    $myplaced = date("m-d-Y", $myTime);

    $params = [
        'from' => $order->getStore()->getEmail(),
        'subject' => $this->t(
        'Regarding your order [#@number]',
        ['@number' => $order->getOrderNumber()]
        ),
        'body' => ['#markup' => $this->t(
        '

Order Summary:<br><br>
Order: @number<br>
Order Date: @placed<br>
Total paid: @paid<br><br>
',
[
'@number' => $order->getOrderNumber(),
'@placed' => $myplaced,
'@paid' => $order->getTotalPaid(),
]
        )],
    ];

    // Set the language that will be used in translations.
    if ($customer = $order->getCustomer()) {
        $langcode = $customer->getPreferredLangcode();
    }
    else {
        $langcode = $this->languageManager->getDefaultLanguage()->getId();
    }

    // Send the email.
    $this->mailManager->mail('commerce', 'receipt', $to, $langcode, $params);
    }

}

I was able to add "Order number" to it, "Order date" and "Total paid" fields, but I cannot add an ordered items list. I tried everything I know, I even tried to implement some recursive function against $order->getItems() to get its fields with no luck. I don't even understand how to see the list of available fields in it. Function print_r gives me memory errors, increasing available memory in php.ini does not help. Looks like any debug info simply cannot fit in email.

How can I get a list of ordered items (Title, Unit price, Quantity, Total price and Image if possible for every order item) and use it in this confirmation letter?

And how can I debug such situations and be able to see content of the complicated variables and objects in Drupal?

PS: I also tried to use EntityTypeManagerInterface this way:

...
    * @param \Drupal\state_machine\Event\WorkflowTransitionEvent $event
    *   The transition event.
    * @param \Drupal\Core\Entity\EntityTypeManagerInterface $entity_type_manager
    *   The transition event.
    */
    public function sendEmail(WorkflowTransitionEvent $event, EntityTypeManagerInterface $entity_type_manager) {
...

But with no luck either, I get this error:

TypeError: Drupal\my_newmodule\EventSubscriber\OrderProcessSubscriber::__construct(): Argument #1 ($entity_type_manager) must be of type Drupal\Core\Entity\EntityTypeManagerInterface, Drupal\language\ConfigurableLanguageManager given, called in /var/www/test/core/lib/Drupal/Component/DependencyInjection/Container.php on line 259 in Drupal\my_newmodule\EventSubscriber\OrderProcessSubscriber->__construct() (line 43 of modules/custom/my_newmodule/src/EventSubscriber/OrderProcessSubscriber.php).