How to do string attachment with wp_mail

It is possible to add an attachment using the standard 'wp_mail' function, however you would need to hook into the phpmailer_init action to do so.

Because this will require calling another function where you don't have any context, you may need to register the action function anonymously with the use ( $attachments ) statement, store the attachment content as a global variable or property, or place the attachment generation code into the new function.

I can see that OP's code seems to be in the context of a class method, so I'll try to create an example which should be compatible / consistent.

    // Previous Code...
    
    // Use this action to generate and/or attach files.
    add_filter( 'phpmailer_init', array( $this, 'add_attachments' ) );
    // And/or store the attachments as a property.
    $this->set_attachments( $attachments );
    
    $sent = wp_mail( $to, $subject, $message, $this->get_headers() );

    // Log errors etc...
    if ( ! $sent ) {
        //...
        $this->log_error( $error_message );
    }
}

/**
 * Add attachments to the email.
 *
 * This method must be public in order to work as an action.
 * @param PHPMailerPHPMailerPHPMailer $phpmailer
 */
public function add_attachments( $phpmailer ) {
    // Remove filter to prevent attaching the files to subsequent emails.
    remove_action( 'phpmailer_init', array( $this, 'add_attachments' ) );
    // Get or generate the attachments somehow.
    foreach ( $this->get_attachments() as $filename, $file_contents ) {
        try {
            $phpmailer->addStringAttachment( $file_contents, $filename );
        } catch( Exception $ex ) {
            // An exception may thrown which would prevent the remaining files from being attached, so we'll catch these and log the errors.
            // This email may be sent without attachments. Do not try/catch if you don't want the email to be sent without all the attachments.
            $this->log_error( $ex->getMessage() );
        }
    }
}

/**
 * Abstraction for the logging code to make it reusable.
 * @param string $log_message
 */
protected function log_error( $log_message ) {
    if ( ! $this->log_errors ) {
        return;
    }
    // Log errors here.
}

This is kind of tricky to do, but it is possible. Write a class which will extend PHPMailer. Override the addAttachment method, so in a certain condition (ex. you make a Boolean class variable) it will call addStringAttachment from the original class and in every other cases the addAttachment class. All you have to do is to set

$attachment = array("your content goes here");

When this is done, make sure that the global $phpmailer variable is an instance of your class and then create the specific condition. wp_mail will call the addAttachment from your class, and together with your special condition the attachment will be sent without saving the file.

Usually I proceed like this

$attachment = array(filepath);

Where filepath is the path to your file.