I've been thinking for a long time about a solo trip, would also like to do a winter trip. But not a solo, winter trip. Margin of error goes way down and you will spend most of your time getting, staying warm. Between that and having groover duty, cook duty and clean up duty EVERY day, there would be little daylight for anything else. Would also need two of everything for warmth/safety (two sleeping bags, two pairs of Sorens, all packed seperately). Swimming or getting gear wet in the winter becomes a survival issue, not just an inconvience. Understand with the current permit system, winter is when permits are available. Also understand the desire to truly have the river to your self. A summer solo trip you would be seeing many people all the time.
One advantage of a solo trip for me would be spending much more time floating the river early morning and in the evening, times typically spent in camp. Would also do many more short hikes. With a big group it is easy to stop at all the standard hikes/attractions, but easy to miss the less popular hikes and side canyons. After five trips I still have a long list of camps and hikes I would like to see. Also could stop at the crowed spots (Matcat, Havasu, Deer Creek, etc) very early or very late to avoid the crowds. Would be no issue with finding camps, there are hundreds of spot where one boat, one cot will fit.
Would greatly reduce the amount of time spent setting up camp and preparing meals. Gear and food would be more backpack style than typical float and bloat. For a summer trip I would sleep on the boat much more often than a spring or fall trip.
My boat would be rigged differently, in that all safety gear would need to be accessed from the top and the bottom. Would travel much lighter than on a big group trip with a much cleaner load, just in case. Despite trying to go lighter, I wouldn't have someone else to cover if both flashlites broke or the security of having 5 plus boats with repair kits/ spare parts.
Just came off of a Sept trip with floats at a steady 10k for 24/7. Would have made a perfect time to run solo because you would not have to worry about moving boat in the middle of the night, or hauling boat back to river if you were a sound sleeper.
A couple nights at Phantom Ranch might be a pleasant break. Warm bed, shower and someone else doing the cooking! Please keep us updated on your plans and hopefully a trip report when you get back.