My Waterford will flex but it immediately dampens out the vibrations. This is what generally makes it a rock solid ride. Once had the front wheel not fully seated, and the Waterford shimmied like my old Bianchi. Once I re-seated the wheel, the Waterford was back to being rock solid again.
This summer I am taking a tour and I am not taking my Waterford. I have been modifying an old steel KONA mountain bike that belonged to one of my kids. The KONA got drop handle bars, bar end shifters, and brake hoods. I also but one of the Schwalbe Marathon tire variants on, maybe 26x1.5. The KONA is not as stiff as my Waterford, but it is a lot stiffer than my Bianchi was. I had problems getting a front rack that worked but I finally ended up with a Minoura steel rack. The Minoura clamps to the brake boss and the axel. The KONA fork is rigid steel, but shaped like a suspension fork and has been a bear to fit a rack to.
You might try a better front rack as I believe that is where a lot of shimmy dynamics start.