I like the idea of forums for the reasons you mention, especially the ability to search and find answers that is often lacking in communication tools like Slack.
I don't mind Slack or Discord, but they can be a time suck, and I find that sometimes I have to back off them for a few days - then it's hard to pick up a thread (especially when people don't respond in a thread!)
I do enjoy the live chat during events, so I like the "office hours" idea for interaction during specific times.
