That's a great question! The reason people say you should focus in when advertising is because this allows you to segment a very small and specific audience. If your audience is too big, then the Facebook algorithm will have a very difficult time finding you good, potentially converting traffic. The same principles apply to the ads themselves.
Unfortunately, I can not suggest an effective way to make a "general" ad for these same reasons. Your best bet is to break down your categories into different ads and audiences. If they are detailed and specific, you can read the data clearly to see what's effective or not.
For example, run one ad for astronomy to an astronomy-specific audience. Run a second ad for the those interested in UFO-related topics and so on.
This is a realistic expense of Facebook advertising unfortunately. You'll need to be specific and do everything separately to get the proper data. Broadening the message will work against you because you might end up not being able to get through to anybody.
Hope that helps add some insight!