Extend MailService and add custom data to every event

CustomMailService.php

<?php declare(strict_types=1);

namespace ExampleMailExtender\Core;

use Psr\Log\LoggerInterface;
use Shopware\Core\Content\MailTemplate\Service\MailSender;
use Shopware\Core\Content\MailTemplate\Service\MailService as DecoratedService;
use Shopware\Core\Content\MailTemplate\Service\MessageFactory;
use Shopware\Core\Framework\Context;
use Shopware\Core\Framework\DataAbstractionLayer\EntityRepositoryInterface;
use Shopware\Core\Framework\DataAbstractionLayer\Search\Criteria;
use Shopware\Core\Framework\Twig\StringTemplateRenderer;
use Shopware\Core\Framework\Validation\DataValidator;
use Shopware\Core\System\SalesChannel\SalesChannelDefinition;
use Shopware\Core\System\SystemConfig\SystemConfigService;
use Symfony\Component\EventDispatcher\EventDispatcherInterface;

class MailService extends DecoratedService
{
    /**
     * @var DecoratedService
     */
    private $decoratedService;

    /**
     * @var EntityRepositoryInterface
     */
    private $customRepository;

    public function __construct(
        DataValidator $dataValidator,
        StringTemplateRenderer $templateRenderer,
        MessageFactory $messageFactory,
        MailSender $mailSender,
        EntityRepositoryInterface $mediaRepository,
        SalesChannelDefinition $salesChannelDefinition,
        EntityRepositoryInterface $salesChannelRepository,
        SystemConfigService $systemConfigService,
        EventDispatcherInterface $eventDispatcher,
        LoggerInterface $logger,
        DecoratedService $decoratedService,
        EntityRepositoryInterface $customRepository
    ) {
        parent::__construct(
            $dataValidator,
            $templateRenderer,
            $messageFactory,
            $mailSender,
            $mediaRepository,
            $salesChannelDefinition,
            $salesChannelRepository,
            $systemConfigService,
            $eventDispatcher,
            $logger
        );
        $this->decoratedService = $decoratedService;
        $this->customRepository = $customRepository;
    }

    public function send(array $data, Context $context, array $templateData = []): ?\Swift_Message
    {
        // fetch my data
        $myEntity = $this->customRepository->search((new Criteria())->setLimit(1), $context)->first();
        $templateData['myEntity'] = $myEntity;

        return $this->decoratedService->send($data, $context, $templateData);
    }
}