WP URL rewrite to follow structure of example.com/{post_type}/{taxonomy_term}/{post_name}

cpt_url_rewrite.php

<?php
/**
 * This assumes you've created a CPT and a custom taxonomy that functions like a tag.
 * You simply attach the taxonomy term to a specific post and voila.
 * Refresh your permalinks!
 * My recommendation would be to use ACF to only allow one tag to be set per post.
 */

/**
 * Filter the custom post type link to create prettier URLs that follow the structure:
 * /{post_type}/{taxonomy_term}/{post_name}
 */
add_filter( 'post_type_link', function( $link, $post = 0 ) {
	$post_type = get_post_type( $post );
	if ( 'content_series' !== $post_type ) {
		return $link;
	}
	
	$post_obj = get_post( $post );
	$post_type_obj = get_post_type_object( $post_type );
	$post_cat = get_the_terms( $post_obj, 'content_series_cat' )[ 0 ];
	
	$post_link = sprintf( "/%s/%s/%s", $post_type_obj->rewrite[ 'slug' ], $post_cat->slug, $post_obj->post_name );
	
	return home_url( $post_link );
}, 10, 2 );

/**
 * Adding rewrite rules for:
 * /{post_type}/{taxonomy_term}/{post_name}
 * /{post_type}/{taxonomy_term}
 */
add_action( 'init', function() {
	add_rewrite_rule( 'content-series/([^/]*)/([^/]*)', 'index.php?content_series_cat=$matches[1]&content_series=$matches[2]', 'top' );
	add_rewrite_rule( 'content-series/([^/]*)', 'index.php?content_series_cat=$matches[1]', 'top' );
} );