For several years, in the 50's, when I was younger, my eastern Sierra three-season setup was an Army surplus poncho shelter, a third of a poncho for a ground cloth, a mosquito head net and a surplus inner bag from an Army arctic sleeping bag. I slept on top of whatever clothes I wasn't wearing. Pitched low, the poncho was good at keeping me dry and the net kept the bugs off. The bag wasn't really all that warm. Altogether this kit and my pack (my "big three") weighed about eight pounds of a total base weight of around 12.5 pounds.

Your idea should work pretty well. A bivy will keep any potential condensation off your bag and the bugs away from your face. You might, however, have to deal with being too warm on occasion. I've never used a bivy so am not sure how one deals best with too warm in one. My thinking would be to get the larger, and flat, tarp. You get more coverage and more versatility with only a minor weight penalty.