It's hard to beat the math. If your tarp is too small to protect against bugs and/or splash back and/or wind driven rain, then you need a weather and/or bug bivy. If you increase the size of your tarp (weight) to the point that it provides complete enclosed bug & weather protection, then you can forgo the bivy. However, either option usually ends up weighing around the same.
Ditto ground cover/bivy. Give up one, increase the other. Ditto quilt/bivy - reduce the insulation (weight) of the quilt, then you need an extra layer (bivy) to provide heat capture.
After you play with the numbers, many typically realize the key is simplicity. So, I generally take a slightly oversized solo tarp (8x8), sans bivy, along with a 4x8 piece of Tyvek.
I've set up my quilt so that I can swap out/combine 2.5 & 5.0 Apex for 50, 35 & 20 degree min temps. I use syn fil instead of down, because I'm willing to deal with more bulk/weight rather than baby my equip. And because I make my own gear, I can swap out/discard the Climashield as necessary as it loses its thermal qualities - even within 12 months - because it's cheaper than the M90 shell/liner.