I'm heading to Quartzite this year for my "once in a lifetime" visit. So I can't comment on the crowds there.
However, I've been to the Keys many times in my life and have camped there often in the winter and I've lived in South Florida for most of my life. I would not worry about crowds in the Keys. As far as campgrounds are concerned, you can expect them to be 90 - 100% full so if you don't like campgrounds that are almost full, you probably won't like campgrounds in Key West.
As far as Key West or other popular destinations are concerned, I wouldn't worry about crowds there either. I've found that the biggest surge of crowds comes from the cruise ships that dock in Key West. As long as you can avoid them, the crowds are rarely unbearable.
If the Big Tent in Quartzite is as uncrowded as Key West this time of year, I'll be very happy.