Wordpress - What's the Best Way to Structure a Multi-Content Blog?

Your sites look nothing alike. Open those three links - and there is not a single element that binds all three sites together. Network of sites can benefit from consistent looks.
You could use global "about" page that covers all of your activity and not just bits in context of specific site. Consider building landing page that will summarize yourself, your network of sites and topics they cover, means of contact, etc. Plus recent activity from Twitter and your network.
Have capable and prominent search through all of the network. It's easy to build with Google CSE and will immensely help people that landed in the wrong place.
Make good use of related posts. Consider what visitor sees when finishing article - is there something that he seamlessly continue to or is it surrounded by completely unrelated posts? On other hand it is probably harder to implement for network, than in context of single site that covers multiple topics...

If I read correctly, it's already a WP-multi site right?

Maybe:

create one main theme that will be used on all the sites
create a child theme for each site that varies the color, or logo, or whatever to make it distinct.
Use common widgets to pull in twitter feeds, etc.
To increase the cross pollination of posts, use a special category that corresponds to each of the other sites in the network. (i.e. in a post on the personal blog, tick the checkbox for the religion blog).
Write a function that extends the regular loop to check the other blogs for content in xyz category. So query_posts() has something like query_posts($query_string.'&extendednetwork=true')

In my opinion, one of the most perfect examples (even when content isn't that far spread) is the landing page of Sergej Müller.

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