Subdomains with almost the same content

Yes, multisite is not what you need here. I would try to map all subdomains on the same wordpress install, and then customize the template via the current URL. In the wp-config.php you then use the current URL as base url:

define('WP_HOME', 'http://' . $_SERVER['HTTP_HOST']); 
define('WP_SITEURL', 'http://' . $_SERVER['HTTP_HOST']);

You can also do different themes per subdomain (see plugins for “theme switcher”).