To me that seems to be contradictory.
Discourse will let non-members read content but they will be prompted to register an account when they want to post a reply. And also after they have read a bit.
But this seems like you are looking for a way to let non-members read content so they will want to register an account, but not let them read content until they register an account. Sorry, but I can’t see how it can be both ways.
How do you imagine this working?