I am likely in a similar boat as you. I would consider myself no more than an average intermediate and I went to Steamboat in the middle of December (15th - 18th, I think) last season. In my humble opinion, much of the mountain is suitable for you. At that time, the mountain was completely uncrowded. There were a few individual trails not open yet and the Pony Express and Christmas Tree Bowl areas (neither of which seem like what you are looking for) were closed. Morningside Bowl opened during the week. Coverage was very good and there was about 4 - 5 ft of snow during the week. From what I have read, Steambaot is considered one of the more reliable early season areas.
I found Wally World to be as you describe. And in December, uncrowded. (At 3:00 one afternoon, I went down one of the trails between Tomahawk and Quickdraw and it only had two other tracks. I had to find a grooming report when I came off the mountain to convince myself the trail was open.) I also had a lot of fun on the intermediates off the top of Storm Peak, especially the trees beside Big Meadow, and in Morningside. I kept off the lower mountain as much as possible, but there are a fair number of groomers there, also. Some of the runs would have part groomed and part bumped. Let me try some bumps with an out if needed. Powder + Bumps = FUN. It did seem to me, however (based on looking at grooming reports while there and then later in the season) that they were not grooming nearly as many runs in December as they were later. There were at most 1 - 2 groomed blacks each day, if any. But it was a powder week, so no grooming could be considered a plus.
I'll throw Grand Targhee out there. From what I have read, about the most consistent early snow you'll find. The vast majority of the mountain is intermediate. But, they don't groom much if your snow really has to be groomed rather than skier packed. There really isn't enough traffic to form many bumps, though. Some think that it isn't big enough for a week, but I did not get bored in four days.