Set Featured Image based on post category - As the post is saved, set the featured image based on the post category: https://www.damiencarbery.com/2018/11/set-featured-image-based-on-post-category/

set-featured-image-by-category.php

<?php
/*
Plugin Name: Set Featured Image based on post category
Plugin URI: https://www.damiencarbery.com/2018/11/set-featured-image-based-on-post-category/
Description: As the post is saved, set the featured image based on the post category.
Author: Damien Carbery
Author URI: https://www.damiencarbery.com
Version: 0.1
*/

// As the post is saved, set the featured image based on the post category.
add_action( 'save_post', 'dcwd_set_post_featured_image_on_save' );
function dcwd_set_post_featured_image_on_save( $post_id ) {
	// Only set featured image for posts.
	if ( 'post' != get_post_type( $post_id ) ) {
		return;
	}
	
	// List the ID of the featured image for each category.
	$category_to_post_thumbnail = array ( 'family-focus' => 3022, 'tailored-training' => 3024 );

	// Skip if featured image already set.
	if ( ! has_post_thumbnail() ) {
		// Extract the category slugs from the returned list.
		$post_categories = wp_list_pluck( get_the_category( $post_id ), 'slug' );

		foreach ( $post_categories as $post_category ) {
			if ( array_key_exists( $post_category, $category_to_post_thumbnail ) ) {
				set_post_thumbnail( $post_id, $category_to_post_thumbnail[ $post_category ] );
				break;  // Set featured image based on the first match found.
			}
		}
	}
}