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 that did a Slack or Slack-like thing did a dual chat/board strat, instead of pretending that chat/email is a serious alternative.