ive never used a bivy before just because of what you describe with the possible condensation. Have you always used one and what are your thoughts of leaving it at home and just rigging up a good tarp system ?
I’ve used both tents (without bivy) and a bivy. The pluses are: it adds to the temperature rating of your sleeping bag (maybe 5 extra degrees). I also like it because you can slip your ground pad(s) inside and you have extra protection from snow. I’ve used the bivy with an overhead tarp and/or winter shelter. I’ve also plopped right on top of the snow with no overhead at all. The downfall is the condensation created from breathing. If it’s not too cold you dont have to zip the bivy fully closed which will prevent the condensation issue (good with an overhead tarp). I’ve only been out once in bitter cold (-20 to -25 deg). It wasn’t miserable to sleep but definitely uncomfortable. I had everything cinched up which produced the condensation, chills, etc.
If I did a winter solo now I would use the bivy with a small overhead tarp if for no other reason than I have it. I like the ground protection it provides from brushing up against or kicking snow in your sleep setup.