Hi Alwyn – my best try at answers to your questions:
“Is interlinking and traffic funneling to and from your related sites a good idea? Do you recommend it? How is this done?”
It’s definitely a good idea, but the challenge is how to do it well. I’ve maintained my sites as separate entities – for the most part – until now, but am starting to do some interlinking by linking from one site to another within articles.
“…what about having a central hub domain where you have some free content etc and then on that site you have links to other domains where the landing page is a sales page and there are no exit links?”
That’s a similar approach to the interlinking one you describe above – but I would stay away from not having any exit links. You want to give your visitors options (and Google doesn’t like it when you box in their customers). Regarding Bob Bly, I’ve heard his name, but don’t really know much about him.
“Do you think that people with a serious health concern have any interest/need in other health issues?”
It depends on the group. Have you asked your subscribers?
“Would it be weird to have two exact same domains on SERP…?”
Nope. I see it all the time.