Apologies in advance, because I'm not caffinated enough to be typing, but here are some of my unwarranted thoughts:
I think that writing is a skill you have to develop like a muscle. Writing something, even if it's just for yourself, is always going to be good workout.
I don't write as much non-fiction, so I don't really know how it works, but I find that when I am writing a first draft of something I don't always know what the focus of a piece will be. It's only when it's finished that I can see a theme and then rework it in.
In terms of writing for an audience, I think that can make writing too abstract. Someone once said that writing for one person is a lot easier than writing for a thousand people, because you cannot always please a thousand people in one go. Just focus on one person when you're writing and how the piece woukd make them feel (This person could be you or a friend, relavtive, loved one or enemy).
Also, if you are concerned that your current essays do not show any connecting theme, show them to a trusted friend or reader. They might be able to see themes in them that you've missed.
I hope this ramble helps and sorry it's not as coherent as I thought it would be.
I'm just about to start Berg by Ann Quin, which seems to be about a man who stalks his dad and mistress through a seaside town. It looks really good from the first few pages.