I use Mastodon Autopost (https://github.com/simonfrey/mastodon_wordpress_autopost) for this. (I think originally came across it via Chris Aldrich). You can choose per post whether you want to post to your instance. I also added a couple of very hacky functions (https://github.com/ngm/semloop/blob/master/includes/syndication-targets.php) to add it as a Micropub syndication target.
