I agree with Mouse: nowadays, a tarp and bivy offer no real advantages over a tent. A weatherproof bivy will weigh two pounds or more; silnylon tarps will add another pound, and “regular” nylon tarps might go two. Good tents don’t weigh much, if any, more than that, and are far more comfortable. I only mentioned the bivies I knew of because the OP seems dead-set on a bivy.

I understand that exposure to high winds might make someone want a lower-profile bivy instead of a tent. I don’t have any mountaineering experience, so I can’t disagree with that. However, in the lowlands, I’ve never had trouble finding a sheltered area to pitch a tent. Even my three-season 2-pound tent easily stood up to an ice-and-snow storm with 30mph winds out in an open bald in the Grayson Highlands one night. I’m wondering how bad things have to get before a bivy is a better choice that a 3 or 4 pound mountain tent. So, with no experience to back me up, I’d probably go with a tent.

But - hike your own hike!