My frustration with the flow, is that you’re forcing me to make a decision at a point where I don’t really know if a thought/idea/comment I want to share will rise to the level of warranting the organizational overhead of making it a “topic” vs just a little toe in the main stream.

I haven’t used Zulip in a while, but can’t you reorganize messages/topics after posting? I remember that as being one of the biggest advantages over Slack for exactly this reason (the Slack equivalent is “I wish I’d known to reply in a thread, because oops, this topic took over the channel”).