In our den with 18 ft. ceilings, we put a VERY LARGE piece of abstract art leaning on the mantel. We also hung our two drapery panels from the highest windows instead of the lower ones like yours. That alone really helped. When we purchased the house, the previous owners hung their draperies like you have them. I don't know why that doesn't work as well, I just know that the higher panels helped. Good luck!

I heard Katherine Hackl speak, and she showed an example of one of her tile installations above a mantle.