I have a trek Allant 7+. When I set the pre-load to minimum I got a terrible shimmy. It seems OK when pre-load is set to max. I weigh 195 lbs. Could the pre-load affect the shimmy like that? It is pretty scary when it happens, even as slow as 20 mph.
If you can move your front wheel side to side with your hand when the bike is on a stand, you should adjust the pre-load so that the wheel doesn't move (side to side). Too much tightness, and you will increase bearing wear...not enough, and your wheel potentially moves side to side. Be gentle with the mechanism, it's usually plastic with a small allen screw.