Don’t worry about the aspect ratio too much if you’re just doing learning exercises, its just if you were aiming for industry work a wider view would usually be expected. The most important thing imo is to get the image working within itself.
If you are going for a more ‘artistic collage’ or digital decoupage as you call it :), then what i’ve put below is not as relevant. If you’re seeking to make a more realistic matte then it might be helpful.
The middle blend stands out for a couple of reasons. Firstly those trees at the edge are quite out of focus which makes the sharpness of the objects behind it seem unreal. Anything behind the far edge of trees should be at least as blurred as they are.
Also the scale of the trees at that point is such that a person standing next to them would be tiny, almost unseen in this picture. The scale of the objects (i can’t really tell what they are, though) behind those trees seem to me much bigger than a person would be at that distance, making your foreground/background transition incongruous.
I think the foreground works (not sure about the scale of the tree though, is it big and next to the house or smaller and close to camera?) and i think the far background works (the silhouetted shapes set against the sky).
It’s your mid-background which is throwing the image out as a whole. I’d replace this with something simple that fits as an extension of the foreground, like just having the trees extend far further back. At this scale any man-made structures are going to be tiny due to the perspective.
Good luck with it, i hope some of what i suggested is helpful 