Is there a way to set default options for Discussions? For example, by default I would like to allow threaded replies, so can I make this the default so I don't have to manually set this every time?
Unfortunately, no, there is no way to set default settings for any type of assignment in Canvas (Discussions, Assignments, or Quizzes).
I did a quick check and found this archived feature idea that sounded similar to what you are wanting - Default settings for discussions. If you are interested you can revive it by creating a new idea - How do I create a new feature idea? - just make sure to link back to the original idea.
See also:Set Default Settings for New Discussions
Retrieving data ...