> One guy in my team would be idle for long swathes of time although it would appear "green" on Teams - first sign someone's an asshole: they put a mouse move program but don't reply for hours.

Maybe his job was not to be on standby for your service 8 hours a day. Maybe he also had a job to do? Why would he be obliged to reply to your messages immediately?

The whole point of Slack / Teams communication is that it can be async, meaning you can ask people something and they will get back to you when it is convenient for THEM, not for YOU. If urgent, just call.