23f/25r GP4000s although I plan to switch to 25f/28r when they wear out. I have a cheap aluminum BD errand running bike with 35mm Vittoria Voyager tires at 50-60psi and the ride quality is pure bliss.

As far as the flex on the Domane there's a promotional Trek video where you can see the seat tube flexing quite a bit. It does make you wonder how it will hold up long term. Although carbon fiber can handle quite a bit of flex without cracking.