I used Divi to build the layout for a custom 404 page. I'm using this plugin https://wordpress.org/plugins/redirect-404-error-page-to-homepage-or-custom-page/ to redirect 404 errors to this page. It works, but when I run tools like screaming frog, it returns broken pages as 301 redirects, which I guess is expected. 

What I would prefer however is for my 404.php template to include the page I built, so that I can still accurately track my 404 errors. I'm thinking of something like:

    #404.php
    <?php 
      
    require(/*Some function to get my entire custom 404 page by slug or ID*/;)

    ?>

What I tried: 

    //Redirect to our custom 404 page
    function wf_404(){
    //Check if custom 404 page exists to protect against infinite loop
       if (is_404() && get_page_by_path('/404-page/', OBJECT)){
           wp_safe_redirect(get_site_url() . '/404-page/', 404);
           exit;
       } 
    }

This gives me a general browser 404 error.