Site meta is a multisite table, it does not exist in the regular single site version. It’s appropriate for your intentions, but then your plugin would only work for multisite installations.
For maximum usability, I’d suggest you use the options table. In order to accommodate a huge number of tags, it’d be worth considering the creation of a custom table.
To use a custom table, all of your table operations would be through methods in the $wpdb global object.
Ok. Thanks very much for the help! I hadn’t realised that was only multisite.