Regarding resting at 7,500m without supplement ox: To be honest, I have no advice for you, it just is the way it is. There is no sleeping that high. That’s just a fact of extreme altitude. I don’t think I’ve ever been able to sleep for more than 30 min at a time above 7,200m or so. Around 7k there is a real threshold for me. I guess everyone is a bit different. I do think it’s possible for people who use a really slow acclimatization schedule to get to the point where they can rest more comfortably at 7K, you’ll see this with the altitude workers because they are such frequent / long-duration altitude dwellers they get acclimated enough to at least sleep lightly.
One thing that will really affect your feeling/experience and ultimately recovery at bivy’s is how wasted you are from that day’s effort. The more exhausted you are, the more miserable the night will be, which becomes a self-fullfilling cycle. This strengthens the importance of your base fitness at the outset.
Spend the time as constructively as possible, usually melting water/drinking it. And know that extreme altitude is a massive sufferfest. Go back and read “Prepare yourself to Suffer” by Jean Troillet on page 384 of Training for the New Alpinism.
Hapy suffering 🙂