I've done what you're describing (kind of)
I'd already bought 11 speed etap for my bike in the last 18 months. But recently got a new frame. Rather than go back to mechanical or spend more money on Di2 I now have the following.
11 speed etap shifters, F+R mech (rim version)
Dura Ace caliper brakes
Ultegra 11s crankset (after what i'd spent on the frame, there was little cash left for the RED crankset)
Yup its a mish mash, but works perfectly.