Replied to https://www.zylstra.org/blog/2019/06/7471/ by Ton Zijlstra (zylstra.org)
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.