I think your idea of limiting Slack is great. You can provide an alternative to be reachable for emergencies if needed.
In addition in my reading, I've come across some helpful nutritional approaches and these were news to me:
http://doctoryourself.com/hoffer_ABC.html http://doctoryourself.com/adhd.html
(These articles are geared towards parents with ADHD kids, but applies to adults also.)