I've got a proper road bike (Trek Emonda - the 'less soft' version of the Domane you're looking at) and a Trek Checkpoint (The trek version of the Cervelo you're looking at). They are more the same than they are different. You don't pay a big price for getting the versatility of the gravel bike - it's mostly small differences and marketing. Most of the components are the same aside from tires, the fork is slightly wider at the top, and rear tire has a bit more room near the bike. Only the air only notices the fork legs are slightly wider at the wider top of the fork, which doesn't cause much drag. The Domane will ride better than either bike if you're running sub-30mm road tires though, but you can make that up with bigger tires. Tires make the biggest difference in speed, so I'd recommend getting a second set of wheels/tires for either bike. With road tires mounted it's hard to tell the difference between my pure road bike and my gravel bike until I run out of gearing following someone fast at 30+mph.
The Domane takes 38c tires, which will take you pretty far soft roading - basically anywhere flat and not sand/mud/snow/rocky. The other bikes will go a size bigger and handle some mud/sand. The gravel bikes have a few more places to attach things, which is nice if you're going rural. I did a road ride in mid-Michigan on a hot day. I used 3 water bottles and the snack bag attachments. I would have needed to taken another route if I didn't have the extra water.
Buy the one you like the best is what I'm saying though. Consider an aluminum version of the bike, as modern aluminum bikes with modern fatter tires basically perform the same as Carbon ones, especially in this category. The softer ride gimmicks on the carbon versions work, but they only turn hard bump impacts into less hard impacts and the flexy seatposts work just as well. I'd steer you to save on the frame and spend more on the accessories (including a second set of wheels, tires, and a cassette - off and on road mode). Like others have said, take a look at the Checkpoint ALR
Last edited by jfranci3; 09-04-19 at 02:33 PM.