SMD Lunar Solo single skin tent.
Lightweight, probably about the same as a bivvy bag + tarp.
Roll the flaps back and it not that much different from a bag with a tarp Flying V behind you. Roll down again it it rains.
Bug mesh.
enough room to move about, get changed, have your essential gear inside with you.
Easier to put up than a tarp. Only needs one pole.
Where a comprise actually turns out to be as good (or at least at times, a better option) as either bivvy/tent.
Not cheap, and get a bearbones pole (not the SMD one).
Oh, I also have a Helium and Siltarp 1.