One way is to set a field called avg rating that can hold num of reviews and total score on the post that the reviews are attached to and whenever a new review is approved to hook into it with acf/save_post (https://www.advancedcustomfields.com/resources/acf-save_post/)
and update the field of the main post with the new values adding +1 to total and +customer rating to the total score.
then on shortcode just do the simple total/num_reviews
