I think the answer here is pretty straightforward: run a Discourse board alongside your Slack, and nudge conversations out of the Slack and onto the board. The board works for long-form stuff, and for stuff you need to reference in the future; Slack works for interactivity when you want the interactivity . We've been doing this for over a year at Fly.io and it's worked out great; I wish every other place I've worked…
Getting serious conversations out of Slack and into a more thoughtful medium seems to be the key to success.
Every time I've seen Slack used as the central repository of communication and information, it's a disaster. No, I don't want to sift through 30 pinned messages in each of the 5 channels vaguely related to a topic to find some piece of information I need. Treat Slack as casual, interactive chat, but get the information and decision-making into a better medium as soon as it becomes more important.