게시물/토픽 열람 권한을 작성자에게만 제한할 수 있나요?

Sounds more like using group DMs. Users can add other users to group DMs.

Admins can control various permissions to categories based on groups, but regular users cannot set individual access to posts, topics or categories - that is what DMs are for.

API documentation:

Official Discourse API documentation: https://docs.discourse.org/