Mine has always been smooth as glass. My gut would also be wheel/tire or confirm alignment. Sometimes spinning the tire a bit on the rim and rebalancing can help but its odd the problem lasted through so many different sets.
Kind of random but maybe the driveshaft is a bit unbalanced? Usually doesn't show up through the front wheel but grasping at straws since you tried so many other things. Its possible to have vibrations at odd mph ranges with it. Couldn't hurt to unbolt it and spin it a 1/4 rotation and see if anything changes.