We’re currently doing this for multiple courses.
We have two approaches that both seem to work out:
For a mathematics preparatory course, we have one permanent Discourse instance that we re-use every year. This makes sense for us, because the course is organized by a team of volunteers, and there is both a large overlap between two consecutive years’ teams, and often a bit to discuss between courses. We archive and tag all topics before the preparations for a new course start. You can read a bit about my journey to do this here:
For other courses, we use one Discourse instance for each iteration. So far, we’ve been simply shutting them down in the end, but in the future, I want to use multisite so I can keep the old instances running without causing significant additional load.
We use a custom course management system that manages submissions, grades, exams and so on. This CMS has a built-in phpBB-based forum, but also supports Discourse SSO – so in the courses where we use Discourse, the “Forum” link in the CMS simply triggers an SSO login to the (login required) Discourse instance for this course. The Discourse instance has a site customization which adds a navigation bar on top, allowing you to get back to the CMS.
I’m sure one large Discourse instance with subcategories could work too, but we’ve made a really good experience with our one-instance-per-course approach.
I’d be happy to answer any questions you have 