Until MS figures out notifications, I won’t be switching. It is hard to understate how annoying it is to not be able to trust accurate notifications and notification settings.
Most messages don’t have an expectation of a timely response. But of those that do, I need to be able to rely on notifications.
Of course Discord has the image problem of being marketed to gamers (and that's fine, because it lets them focus their efforts), but as a tool it is pretty well crafted.
I just gave up eventually.
Have you spoken with Slack directly about this? I've brought up several issues to their "Feedback" area and usually have very quick responses for a company that services as many people as Slack does.