WordPress custom post navigation inside of a shortcode

custom-post-navigation.php

<?php

function custom_post_navigation_function() {
  ob_start();

  // Setup and store previous post variables
  $prev_post  = get_previous_post();
  $prev_id    = $prev_post->ID;
  $prev_title = $prev_post->post_title;
  $prev_link  = get_permalink( $prev_id );
  $prev_thumb = get_the_post_thumbnail( $prev_post->ID, 'thumb', array('class' => 'previous-post-image'));

  // Stores previous post markup in variable to be called later in the function
  $custom_previous_postnav  = '<a href="' . $prev_link . '" class="custom-previous-post_wrap">';
  $custom_previous_postnav .= '<div class="custom-previous-post_image-wrap">' . $prev_thumb . '</div>';
  $custom_previous_postnav .= '<div class="custom-previous-post_content">';
  $custom_previous_postnav .=	'<div class="previous-post-sub-title">« Previous</div>';
  $custom_previous_postnav .= '<div class="previous-post-title">' . $prev_title . '</div>';
  $custom_previous_postnav .= '<div class="custom-previous-post_content_read-more-link">Read more <span class="right-arrow">→</span></div>';
  $custom_previous_postnav .= '</div></a>';

  // Setup and store next post variables
  $next_post  = get_next_post();
  $next_id	  = $next_post->ID;
  $next_title = $next_post->post_title;
  $next_link  = get_permalink( $next_id );
  $next_thumb = get_the_post_thumbnail( $next_post->ID, 'thumb', array('class' => 'next-post-image'));

  // Stores next post markup in variable to be called later in the function
  $custom_next_postnav  = '<a href="' . $next_link . '" class="custom-next-post_wrap">';
  $custom_next_postnav .= '<div class="custom-next-post_content">';
  $custom_next_postnav .=	'<div class="next-post-sub-title">Next »</div>';
  $custom_next_postnav .= '<div class="next-post-title">' . $next_title . '</div>';
  $custom_next_postnav .= '<div class="custom-next-post_content_read-more-link">Read more <span class="right-arrow">→</span></div>';
  $custom_next_postnav .= '</div>';
  $custom_next_postnav .= '<div class="custom-next-post_image-wrap">' . $next_thumb . '</div>';
  $custom_next_postnav .= '</a>';

  // Wrap both prev and next nav in a container and output markup for both
  $custom_postnav = '<div class="custom-post-nav d-flex justify-content-between">' . $custom_previous_postnav . $custom_next_postnav . '</div>';

  // Outputs all markup
  return $custom_postnav;

  ob_get_clean();
}
add_shortcode('custom_post_navigation', 'custom_post_navigation_function');