I'm not an expert but when I was looking at bivies I was surprised at how different they were.

The very light ones are not waterproof, just splash resistant. These tend to be made out of light, water resistant fabrics like Epic. They are designed to be used in connection with a tarp. Others are closer to waterproof and are made of the same kind of stuff a rain coat is made of. In these type of bivies the concern is more about getting wet from condensation than from precipitation. In that way they are more like a small tent but with even less circulation.

Many bivies, but not all, have a bug screen that you can zip that will totally protect you from the bugs.

I own an inexpensive bivy that I like. It's an REI minimalist bivy. Very cheap for what you get. It has bug netting built in. Quite waterproof except that between your face and the sky the only thing is the bug netting. When I use it I just drape my raincoat over that opening if it starts to rain or snow. That solution isn't very stormproof though since a strong wind would carry my raincoat away. I mostly use it in snow conditions. I think winter is really the time for a bivy as it adds considerable warmth to your bag rating.

If you aren't that concerned about your bag getting wet (like the bag isn't made of down and has a water resistant outer) then I would be strongly tempted to just use a tarp rather than a tent and forget the bivy. You can't get lighter than that. And if you're expecting a storm you can pitch it really close to the ground and windblown rain isn't much of a concern (provided the tarp isn't tiny). If you use trekking poles then you always have something to pitch it with as well, even in an area with no trees. I guess running out of trees seems unlikely on the way to Alaska, buy hey, I don't know, I've never hitched to Alaska.

Have fun!