Great shots, very nicely done.
Maybe next time move to the other end of the ring so there is the greatest distance between your subject and the outside edge of the ring. A long telephoto lens will "compress" the distance between you and the subject, while the angle will mean the subject is farther away from the background (with limited DOF at wide open aperture the background will be blurred).
Shoot at full zoom longest focal length with widest aperture to make the background blur the most and fill the frame with subject.
That way everything in the frame that is important (horse, rider and barrel) is in focus and everything else is blurred or out of focus.
Here is my example:
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