I’m not sure if you’re already aware of this option - apologies if so.
The Slack API has a section on “Slack App Unfurling”. From a brief read it looks like you can do some clever stuff with the API which replaces slack’s default unfurling for specific links with your own logic. Maybe discourse-slackdoor could be adapted to use this method, rather than the existing “sends a message in response” method?
Using this API would also mean users can put the link anywhere in their message, rather than only at the beginning.