Dynamic Endpoints

I have a database table outside WordPress that I need to make endpoints for. I’ve created a page /cars/ where I plan on using a page template to generate links. I would like for the url to look like /cars/camaro/ ( /cars/%model%/ ). Originally I thought I could do it with endpoints but wasn’t sure how to make them dynamic based off of the slug pulled from the Model Table. I also wasn’t sure if it would be easier using Permalink Structure tags or which was better.

I’m not even entirely sure where to start, I’ve created “static” endpoints before so I had a base to jump off of but when I get to the request filter I’m not sure what to do.

/**
 * Add endpoint to handle models
 */
function theme_endpoints() {
    add_rewrite_endpoint( 'cars', 'EP_PAGES' );     // Since cars is our base
}
add_action( 'init', 'theme_endpoints' );

/**
 * Filter request variables
 * @param array $vars
 * @return array $vars
 */
function theme_perma_requests( $vars ) {

    if( isset( $vars['model'] ) ) {
        $vars['model_template'] = true;
    }

    return $vars;
}
add_filter( 'request', 'theme_perma_requests' );

I mean, this allows me to my template_include ( which is why I added the model_template variable ) but it doesn’t actually rewrite the URL. So then I started playing with add_rewrite_rule()

/**
 * Theme rewrite rules
 */
function theme_rewrite_rules() {
    add_rewrite_rule( '^cars/([^/]*)/?$', 'index.php?page_id=8&model=$matches[0]', 'top' );
}
add_action( 'init', 'theme_rewrite_rules' );

I’ve flushed my permalinks but the rewrites don’t seem to take effect, I can only assume something’s wrong with how the rule is written.

Do I need both the endpoint and the rewrite rule or just one of the two? Am I going about it all wrong? How can I achieve the permalink structure of /cars/camaro/?

10up engineering best practices as a great example on how to achieve that.
You basically add a new rewrite tag and a new rewrite rule and then using the template_redirect action you return your custom action.

Here is the code:

add_action( 'init', function() {
  add_rewrite_tag( '%model%', '([^/]+)' );
  add_rewrite_rule( 'cars/([^/]+)/?', 'index.php?model=$matches[1]', 'top' );
} );

add_action( 'template_redirect', function() {
  global $wp_query;

  $model = $wp_query->get( 'model' );

  if ( ! empty( $model ) ) {
    // Return stuff here
  }
} );

Don’t forget to flush permalinks before trying your new endpoint!
Using wp-cli:

wp rewrite flush

Or using the admin UI: http://example.local/wp-admin/options-permalink.php and click save.