The current Synapse is designed to accept 32mm tires max. Not sure about the mid-teens disc model, but it certainly won't be larger than 32.

A 32 tire is fine for me on smooth hardpack gravel and dirt roads. It'd be no fun at all on any other gravel roads. Around me it seems like the gravel roads change in composition every 1mile or so. It'll be smooth hardpack, then hardpack with loose on top, then back to hardpack, then soupy loose, then more hardpack with loose on top. Etc etc.

For Cannondale, the topstone would be what I would consider in riding 60% unpaved. The trek domane has a huge amount of clearance for whatnis considered a paved road bike.
...but really I would just find a gravel bike with the geometry I like and call it good. A lot of my rides are 60%gravel/40%paved as I ride paved roads to start and finish most rides. My gravel bike has frame angles, trail, and chainstay that's very similar to my main road bike.