I like forums. Always have. I appreciate threading and searchability. They are both immediate and long-lasting.
I'm not a fan of Slack, nor of Discord. Slack I just find inconvenient for long content-rich discussions. I had a bad experience with a Discord channel that was set up with insufficient security, and I'm hesitant to go back.
I'm not sure how the chat (cbox) integration would work, or if it would be of benefit outside of the way it's used now when it accompanies a live training session, but I'm willing to try!