Firstly -- You've been doing decent so far. Pretty impressive for a first venture, if I do say. I've no real hands-on experience with sequential art / graphic novels either, but your picking / choosing has worked for me so far given the type of story you're conveying and the quality of art you use. I'd highly recommend you pick up a copy of "Watchmen" (graphic novel version, of course) if you wanted to see perhaps the single best use of how to establish these things. In the meantime, methinks a grand rule of thumb is to always imagine that it's the exact same as a film - direct the shots and the action on screen as you would a movie; however you'd love to see a film version of it done is how it should be.
Secondly -- I don't know (for) how long you plan on working with this story, so I can't really speak on pacing yet. I've always been of mind that, since it's a world which you yourself fully control, nothing that has no place in it doesn't need to exist. To me, all things should advance something else -- every image, dialog, and action -- be it a plot or a character. Again, just a rule of thumb I myself ascribe to.
Thirdly -- Personally, I'd enjoy vignettes. I'm a fan of appendices, footnotes and the like -- they're both self-contained and expansive to the universe in question without having to bog down the narrative unnecessarily. Avoid exposition unless it's absolutely critical and narrative unless it's something that the character would actually say / have a reason for saying. Remember: this story is told as a tale being told, so you have yourself a little more leeway than you would if it was told in a different style.