My view has always been, if you need a wr bivy to stay dry with tarp, your tarp is too small.
I use a bug bivy or solo net tent with 8oz shaped tarp. For me, combo works great.
Bug bivy or narrow net tent can be used in shelters , by themselves, or with tarp. Total protection and flexibility for 13 -18oz.
If there were better system at any cost, id be using it.
Only thing I don't like is the pole is outside the net tent when the net tent is pitched under the tarp. I like to hang my pack from my pole to keep it off the ground and out of the way. When I hang it outside the net tent it kind of swings into the net tent and encroaches on room. This is because the pole leans at an angle. The vertical pole inside my Hexamid is much better in this in that regard.