I've run the gammut of very light options of: bivy/tarp, dcf 2p tent, tipi and 16oz 1p. For carrying on my back like you mentioned, my current preference is the 1p Tarptent Protrail Li, at just 16oz.
For me it's the best compromise of being as light as a bivy/tarp, having more space than other 1p uberlight 1p tents, quick setup, pretty small rectangular footprint and fully enclosed with floor.
I've found that I sleep best with tent fabric further from my head, vs right on my face in a bivy, or super close with the other light 1p or tipis I've tried. You have to shimmy into the Protrail from the head door, but for hunting (especially archery) I don't go in-out much at all, so I've found I prefer to sacrifice that for the better sleep and comfort I get from the space above my head when sleeping and time in the tent.
The biggest bivy/tarp benefit I see is the ability to simply unroll anywhere. If you can sleep with fabric on your face, then that's a pretty sweet time and space benefit.
I see the Protrail as my bivy/tarp alternative for basically the same weight.
However, for me personally, that is not enough to endure the poorer sleep I get in them. Maybe one gets used to it, as I admit I didn't give it a whole heck of a lot of nights before I switched. I haven't done it yet, but in a pinch for space I do wonder if I could use the protrail like a bivy, without stakes and just having a ton of extra material.
Here's a pic of it next to my buddy's bivy/tarp setup. Both are very cool to utilize.
View attachment 1065767
Can check out their site for the footprint dimensions:
The ProTrail Li is our least expensive and simplest Dyneema® shelter. This tent is ideal for trekking pole users who want the ultimate lightweight and protection. Simple, sturdy, for a wide range of conditions.
www.tarptent.com