overwrite the_content() with WP template output

plugin.php

<?php 
/* Retrieve the rendered content generated by Advanced Custom Fields and your template and 
 * save it to the_content field. Overwrites existing content. Useful for Yoast SEO analysis compatibility 
 * and acts as a fallback if the user ever changes themes.
 */

/* Requires simplehtmldom, get it here: http://simplehtmldom.sourceforge.net */
require 'simplehtmldom.php';

add_action('acf/save_post', 'mctc_acf_save_post', 1);

function mctc_acf_save_post($post_id) {
    if( empty($_POST['acf']) ) return;

    /* Works with TwentyFifteen. CHANGE THIS to work with your theme. */
    $contentID = '.entry-content';
    
    /* Get the Preview URL */
    $post = get_post($post_id);
    $preview_link = set_url_scheme( get_permalink( $post_id ) );
    $preview_link = esc_url( apply_filters( 'preview_post_link', add_query_arg( 'preview', 'true', $preview_link ) ) );
    
    /* Use the auth cookie of the currently logged in user to retrieve the page content */
    $cookies = array();
    foreach ( $_COOKIE as $name => $value ) {
        $cookies[] = new WP_Http_Cookie( array( 'name' => $name, 'value' => $value ) );
    }
    
    $request = wp_remote_get( $preview_link, array( 'cookies' => $cookies ) );
    $body = wp_remote_retrieve_body( $request );
    $html = str_get_html($body);
    $primary = $html->find($contentID,0)->innertext;
    $post->post_content = $primary;
    
    /* Unhook so we don't get an infinite loop */
    remove_action('acf/save_post', 'mctc_acf_save_post', 1);

    wp_update_post( $post );

    /* Rehook */
    add_action('acf/save_post', 'mctc_acf_save_post', 1);
}