I'm glad you went ahead and finished this top. I think you're going to like it when done.
I would play around with some ideas for a while. Print out full page photos and doodle a bit to see what you like.
I am thinking rrganic linear quilting (meaning lines, but not worried about being super-straight or evenly placed) in thread matched to fabric (I'm not a fan of contrasted thread unless you are going for a very specific look). I'd want to contrast the two background segments -- maybe one horizontal lines, the other vertical. Or one pebbled, the other lines. The frames could do with some strong pattern to set them off. Inside the boxes I think I would repeat the same approach each time or at least the same for all the boxes of same color --not doing a different motif in each box-- that would just be distracting from what is a strongly graphic piece. I'd probably go for a "transparency" effect by continuing the same quilting approach used on the matching fabric of either side (so if I did black pebbles on the side, I'd do pebbles in the black box.) -- that would make it look like you had interlocking windows framing 2 sides .
For me, whatever happens on the back is a happy accident, so I wouldn't spend too much time debating how the quilting on the front will play out on back. Usually I match bobbin and top thread unless I want to use a fine thread like bottom line in the bobbin, and then I would go with something that will blend really well with everything.