Hmmm....flat bar and "light" are going to be the problematic issues. Most all "gravel" bikes are dropbar with brifters....and those that aren't are usually 4130 to 525 level steel. 45mm tires with room for fenders means 2" clearance either in 29x2" or 27.5x2"


Have you considered the Bridge Club?


https://surlybikes.com/bikes/bridge_club/bike_specs


I see you already saw the Salsa...