You can use wp-discourse to control Discourse logins with WordPress. It can be configured to pass group membership as well. You can use various membership platforms; each has its own rewards and challenges. I don’t know how to combine that with LearnDash.
See WP Discourse Plugin Tips and Tricks and WP / MemberMouse / Discourse Integration Guide, Sync WooCommerce Memberships with Discourse groups[Managing Discourse group membership with WP Discourse SSO],(Managing Discourse group membership with WP Discourse SSO), for example.