Currently, MailBeforeSentEvent has no setter methods to manipulate mail data.

The Symfony documentation describes such an event where one could update subject and message properties: https://symfony.com/doc/current/event_dispatcher.html#how-to-customize-a-method-behavior-without-using-inheritance

My use case is that I want to edit the mail contents. For that, I need to adjust the return value of the private method "buildContents". That seems not possible with a decorator.

A workaround is to completely replace the core MailService class:

<service id="Shopware\Core\Content\Mail\Service\MailService" class="MyPlugin\Service\MailService">
...
</service>

That, however, will lead to breaking changes if the MailService class gets patched in the future.