Students need specific guidance and support for how to develop, reject, and refine ideas appropriately in your course.
If you want students to share well, consider requiring an initial post where you and students introduce yourselves and share a picture.
Describe your expectations for norms in how everyone will behave online
Provide a lot of initial feedback about the quality of posting. Consider giving samples of good and bad posts, and remember to clarify your marking criteria. Focus your expectations on the quality of comments, and set maximums for the amount you expect to reduce your marking load and keep the discussions high quality.
Someone will need to moderate the discussion. That includes posting the initial threads, reading what everyone posts all weeks and commenting to keep the discussion flowing. Likely, the same person (you or a TA) will also be grading and providing private feedback to each student. Consider making the moderation of a discussion an assignment in your course. You can moderate the first few weeks to demonstrate what you want, and groups of students can moderate other weeks. It can increase engagement if done well, and definitely decreases your work load.