I really like this proposal, Discourse is clearly a great (if not the best) choice for providing a forum experience now.
However, currently (as far as I know) there is no official WYSIWYG editor plugin for Discourse, the users need to use the Markdown based editor that is used on this Discourse instance as well.
I don’t know if this is a problem at all, I really like the current editor of Discourse, but I don’t have a problem with writing Markdown 
There was one WYSIWYG editor plugin developed for Discourse a few years back: https://github.com/procourse/Discourse-BEditor
It hasn’t seen many updates, but since it is built on top of React, it could be used not just as an editor on the Discourse site, but maybe as an embedded editor directly in Open edX as well.