Congrats on your new bike. Looks great. I love the cargo aspect of the bike.
Shimano Di2 + STEPs. Nice combo, I used a similar config all summer.
A tip for traffic: Just before stopping at a red light, drop a gear or two and switch into High assist mode. When the light goes green, accelerate up to cruising speed, and then switch back into Eco mode. You'll get off the line quick, get some exercise, and still get decent mileage. Unlike some other mid-drives, this is easy to do because of the cockpit where everything falls under your thumbs, and because you're assisted by electronic shifting.
I wasn't really fond of the STEPs drive when I first tried it, but I started to like it more as I used it and compared its behavior to other mid-drives systems (I rode about 1000 miles on the STEPs). What's really great about this system is that the power delivery is extremely predictable. If you drop a gear and pedal at a slightly higher cadence, the power immediately kicks in. On some of these mid-drives, it's difficult to know what to do in order to get the most out them.
Anyway, have fun. And let us know about your experiences.