I understand your points.
Would outsiders by more likely to read and participate in the notebooks section, more than the forum? I guess the quality of writing and the writers would attract other writers from outside of the site (if that’s what you mean). The key is that they have to show a willingness to engage with people who comment on the articles—at least that’s my opinion. If that happens, I think I would be attracted to the notebook section. At the Atlantic website, some of the writers will interact with the readers on their blogs and it’s an appealing feature.
I also understand the concern over spending time on threads, only to have them fade into the background quickly. Since I know you put in a lot of thought and effort into your posts, I can understand your reluctance even more. Still, would you consider writing shorter posts, ones that don’t take so much time and effort—or would that make posting less appealing? I’m just trying to think of a way you can get around this hurdle. I think if you quickly jot down some comments, that should be sufficient to start a conversation. Imo, the OP is not really crucial. It’s the participation afterward that really matters, so I don’t think you need to spend a lot of time and energy on the OP (if that is the concern; I do think facilitating a discussion takes time and energy, though).
It’d be nice to see you starting some threads, in any event.