you could try using timewarp to try and add motion blur to the footage.
add timewarp prior to the echo effect. set the speed to 100 (you don't want to change the speed), then go to the motion blur settings and see if you can add enough motion blur to soften the hard circles.
there are third party effects that can do a better job of creating motion blur. revisionfx's reelsmart motion blur is pretty good. if timewarp can't do it well enough, i think they have a free trial you could try.
this may not help you now, but if you know before a shot that you would want to do this type of frame blending effect, and you want the smoothest blending, try shooting with a slow shutter speed (as close to the frame rate as possible) or high shutter angle (as close to 360 as possible).
this should give you the greatest amount of motion blur to the original footage, which will help create the smoothest blending of frames.