If it’s not significantly more work up front, also federating out post edits as Update Activities would be valuable. If it’s one of those things that can be contributed post-MVP, then a design that reasonably accommodates adding that later would be great.
For what it’s worth, I would set the delay low on my instance(s). I would not plan to use it to avoid federating spam posts, because it would reasonably likely be a signal that brings a mod from their fediverse feed into our Discourse to take action. I have chat_integration_delay_seconds set to 20 for quick typo edits, and would expect to do something similar here. At the very least, that setting sets a precedent for such a delay. 