To deploy a poncho tarp without a second piece of rain gear means you have to be committed to getting wet or rain is not that big an obstacle. Once you have the poncho tarp deployed that’s all there is.
Second, deploying a poncho tarp means having to attach and detach cordage exposing you to more of the same.
Third, When deployed as a tarp the poncho tarp is smaller giving you less protection. The largest tarps without a seam is based on the manufacturing process and is currently 6 feet wide. Take some material for the outside seam and you get to about a 5.5 feet. My poncho tarp is just over 4 feet wide.
Lastly, the lightest poncho tarps are fragile as are the lightest rain jackets. So if you damage the poncho you damage both. Where-as if you damaged your poncho you could always wrap yourself in your tarp.
Actually there is plenty of room and the bivy offers some additional rain protection, however, the half pyramid configuration does not get the mosquito netting off my face. One possibility is adding an umbrella to the open side.
The umbrella by itself did not add sufficient strength to the to hold up the head end of the bivy.
If you look closely at the head end there is a small pole holding up the end… there is also a guy line adding tension/structure. At this point the head end is slightly more exposed on 2 sides. The umbrella can be used to handle the rain. Also the pole can be replaced with a branch or even a tree or bush.
Not pictured here there is another option. That is some cordage from the the pole to the lower corner at the head end. The with a small loop and an s-biner or other lock would work fine. The umbrella would still offer some protection for the head and the pack.
Even though this configuration is plausible it still has the eggs in one basket challenge.
I added the diagonal guy lines and I was able to get the bivy to blouse nicely, however, it was 5-15F hotter inside the bivy. Also being so low to the ground meant being more uncomfortable. All those aches and pains are amplified.