Regarding your desire for a gravel tire with a "solid" (smooth) center tread - I've tried a few if these types of tires, with small knobs on the side of the tread and a smooth center. I find that they are very buzzy on pavement, and more importantly are terrible at keeping traction while cornering, because the knobs don't support the tire well at an angle. Some examples are the WTB Byway and the Simworks Homage (based on an earlier Panaracer design). I would stick with a wide but smoother tread tire or one with small knobs all the way around.
The guys at Bicycle Quarterly mention this when talking about development of a knobby gravel tire:
https://www.renehersecycles.com/the-...pose-knobbies/