WP (SSO) + LMS + Discourse Advice/Best Practices?

From your onboarding list, it looks like you are hoping to override the Discourse trust level system. I think a better approach would be to use Discourse group membership to control access to your site’s content. For details about how this works, see How to use category security settings to control access to content.

The WP Discourse plugin has a couple of functions that you can use to add and remove users from groups. Details are in this topic: Managing Discourse group membership with WP Discourse. That topic uses the PaidMembershipsPro plugin as an example, but the same idea should work with MemberPress.

Another topic that may have useful information is How to prevent some WP users from being able to login to Discourse.