There's a difference between saving as much weight as possible and as much weight as practical. Given your needs, a bivy will save you a little weight but cost you a lot of convenience. If bugs or even light showers are possible do you really want to be confined in a bivy "coffin" for hours on end?

Everything but sleeping is much more of a hassle in a bivy. How will you change your clothes? Where will you hang out if it's raining? What will you cook under?

Besides, some bivies weigh more than some tents. As mentioned above it's best to have a tarp with a bivy. That requires stakes and guylines. And you may want a ground cloth under the bivy. All this stuff adds up to almost the weight of a tent and without the weather/bug protection.

Bivies have their niche. I own and use them myself. But for your situation they're not the best choice. I think you'll be much happier in a tarptent.