Just turn off Slack notifications. I did this years ago (as a founder), and my life improved considerably. Three people at the company had my personal cell number. If it really was an emergency, they'd call. It only happened twice. Both times were legitimate emergencies that I would've wanted to know about ASAP.
After a few months of turning off Slack notifications I started telling all my employees to do the same. Most did. Everyone was happier.
> I firmly believe that good management can overcome these issues, but it must come from the top.
It starts at the top but is really implemented by middle management. As a leader, you have to get middle management on board with it, or some teams will end up with the short end of the stick. I had one team lead that would Slack their employees all weekend long and explicitly wanted a reply. Once I got word of this, I started calling that manager every single Sunday until they "got it".