Yo, I really like your linework, very solid!
Your colored pieces are also looking good. I am not too familiar with the theory behind composition in large vertical pieces (though some of my favorite pieces of traditional art are in this format)
From what I can tell it is quite balanced to my eyes. I tend to gravitate my view to the middle, maybe the background at the top is a bit busy and the level of detail I find to be a bit too homogeneous - I think varying LOD with some more detailed plants maybe a single flower and lowering a bit the LOD on the farther background can help
cheers