I have a Cannondale Synapse and love it. Mine has 105 on it but I did swap out the wheelset for a carbon wheelset and replaced the seatpost, seat, stem, and bars. I'm also planning on replacing the groupset with Di2 this year hopefully. I'd pick it over any of the options you listed (I don't want a Bianchi).
A Synapse with Ultegra Di2 for $3,800 shipped sounds like a great deal to me.