Only thing that changed was the tire.
Short of having developed a mechanical issue like loose steering head bearing (it’s checked and adjusted as part of the 25k mi service), bearing hub, (doubtful in 20k mi, but if you use a power washer on those wheels...) , axle nut torque, or front end suspension issue (fluid), These are probable but I’m thinking not likely.
That “light” feeling could have even come from the road condition. Don’t imagine you’d go that fast if it was, but did the road look so smooth that it looked “slippery”? A slippery road would make the front end feel light at that speed (as would a worn front tire).
The shimmy when applying the front brake, did you feel a pulse in the brake lever, or are you saying the front end shimmied side to side, but the braking felt smooth? If the latter, you’ve got slop somewhere which might also cause that light feeling, but if you felt it in the lever, probably a warped rotor, or as Reds4 states glazed.. If it’s a rotor (warped or glazed) you would always feel it in the lever when applying the brake regardless the speed you’re going, it’s just more pronounced at higher speeds.
My guess is 1. worn front tire, 2. mismatched (with worn front) tires, and 3. warped rotor.