This hook let's you redirect to a custom page when a LearnDash course is completed.

learndash-course-completed-redirect.php

<?php

function custom_course_completed_redirect($link, $course_id) {
  $link = get_post_field( 'post_name', $course_id );
  //You can change the link here
  return '/congrats/' . $link;
}

add_filter("learndash_course_completion_url", custom_course_completed_redirect, 5, 2);

?>

Can this be revised to redirect a lesson?