Get woocommerce email template as string

get_wc_template_email.php

<?php
/**
 * Get WooCommerce Template Email
 *
 * Ref : https://gist.github.com/craigedmonds/b65c65e02fd40bd381bf023ffef2c80e
 */
function get_wc_template_email($content, $heading = false) {
    $wc_mail = new WC_Email();
    $message = wc_get_template_html('emails/custom-content.php', [
        'content'           => $content,
        'email_heading'     => $heading,
        'sent_to_admin'     => false,
        'plain_text'        => false,
        'email'             => WC()->mailer()
    ]);

    return apply_filters( 'woocommerce_mail_content', $wc_mail->style_inline($message) );
}