Set CPT slug by default based on taxonomy and post_parent

default-event-recap-slug.php

/**
 * Default event recap slug
 * If has parent and is in category Recap, set slug to "recap".
 * Can't use `wp_insert_post_data` because taxonomy data isn't included, frustratingly.
 */
// Catch changes in block editor
add_filter( 'rest_post_dispatch', 'recap_slug_rest', 10, 3 );
function recap_slug_rest( $result, $server, $request ) {
	if ( 'PUT' == $request->get_method() ) {
		$post_id = $request->get_param( 'id' );
		$post = get_post( $post_id );
		if ( 'event' == $post->post_type && $post->post_parent ) {
			$terms = get_the_terms( $post_id, 'event_category' );
			foreach ( $terms as $term ) {
				if ( in_array( $term->slug, [ 'recap', 'recaps' ] ) ) {
					wp_update_post( [ 'ID' => $post_id, 'post_name' => 'recap' ] );
					break;
				}
			}
		}
	}
	return $result;
}
// Catch quick/bulk edit
add_action( 'save_post_event', 'default_recap_slug_save_post', 10, 3 );
function default_recap_slug_save_post( $post_ID, $post, $update ) {
	if ( defined( 'REST_REQUEST' ) ) return;
	if ( $post->post_parent ) {
		$terms = get_the_terms( $post, 'event_category' );
		foreach ( $terms as $term ) {
			if ( in_array( $term->slug, [ 'recap', 'recaps' ] ) ) {
				remove_action( 'save_post_event', 'default_recap_slug_save_post' );
				wp_update_post( [ 'ID' => $post->ID, 'post_name' => 'recap' ] );
				add_action( 'save_post_event', 'default_recap_slug_save_post', 10, 3 );
				break;
			}
		}
	}
}