"Do you want to use bivy in the rain, or under tarp?"
That is the important question.
If it will be used directly out in the rain, then you need a fairly high degree of waterproofness. If it will be used underneath a tarp, then you need just a bit of splashproofness.
My idea, not yet proven out, was to use a lot of mesh in the top for maximum breathability. Then add a "rain flap" which would cover over the mesh, just for a rain storm. The mesh would still allow a bit of breathability under the rain flap.
The rain flap could be made out of cuben fiber, for minimum weight, or out of silnylon for less cost.