Possible to automatically create forum topic when user submits review on Wordpress theme?

It’s hard to say. One approach would be to publish the ‘listing’ posts to Discourse and have the reviews created directly on Discourse as replies to the post. Possibly the Topic Ratings Plugin could get you started with this. It is possible to display the ratings that are created with that plugin in comments on WordPress. It would take some custom code on WordPress to display the ratings though.