Add reading time to theme with a proxy function.

functions.php

<?php
namespace PublicFunctionOrg\Theme\ReadingTimeSample;

// ...
function reading_time($post_id){
  if(function_exists('reading_time_render')){
    reading_time_render($post_id);
  }
}

single.php

<?php
get_header();
// ... all the html page rendering
while ( have_posts() ) :
  the_post();
  // ... post html rendering
  
  \PublicFunctionOrg\Theme\ReadingTimeSample\reading_time(get_the_ID());

  // ... even more post html rendering
endwhile;
// ... even more html page rendering
get_sidebar();
get_footer();