Based on my experience I would say go with a Domane (SLR, since your budget allows it) or a Cervelo R3.
The Domane is a very comfortable ride and can handle tires up to 38c (I believe), and is pretty lightweight for an endurance bike.
The R3 is also very smooth over rougher roads...for a race bike. It's not Domane smooth, but it doesn't beat you up like other race frames. And you can get it with Ultegra Di2 well under your budget. The geometry is also slightly less aggressive than most race frames, which helps on the long days, and it can handle fairly wide tires. I had 28c GP4000's (which measured 31.5mm) for a while on mine, and there was still room to go wider.