How do you have deep, written discussions with your teams?

What is the best way to have careful, deep, written discussions among distributed teams? Are there tools meant for this? Would something like a BBS work. If so, are there good modern options?

We use distributed teams. We communicate using a mix of email, Slack, and Zoom. Our work involves lengthy discussions and deep dives into complex issues. This type of 'deep discussion' benefits from carefully written arguments and counterarguments.

Emails often start fine. Someone sends a well structured, well written argument. The first few replies will be strong. But then it diverges into a mess of threads that are hard to follow. People resort to color-coding their responses in-line, etc.

Slack is too chatty. Other chat-based solutions are the same. I've never seen it work for this type of 'deep discussion.' Conversations get scattered across channels and threads within channels. Maybe we're using it wrong. To me it's the worst way to encourage deep discussion.

Zoom, calls, and in-person meetings are hit or miss. The advantage is they seem to cut to the chase on simple issues. But for deep discussion, they often go nowhere. They favor speaking ability. No one prepares enough. Instead of careful thought and discussion, you get hot takes. A lot of our discussion benefits from going away to gather evidence or think more. Rarely is there a need for synced discussion. And there's never enough time.

I've tried to find off-the-shelf solutions. A simple, old-fashioned BBS seems best. It breaks things into the right unit of discussion. It works for short- or long-form discussion. It creates a coherent timeline of discussion, etc.

I worry that without the bells and whistles of a modern app, getting the team to use it will be a challenge. It'll be viewed as a stale company discussion board. A lot of options I've looked at have clunky interfaces and tough learning curves for what should be super simple and intuitive.