The biggest drawback to using a bottling bucket as a secondary may not be the amount of headspace, although that is a huge concern. I would never do it. If I was using a clearing vessel ("secondary" isn't the correct term- it's either a clearing vessel or a "bright tank"), I'd use a carboy. A bucket is very wide- that's the headspace I'd worry about, not the depth. A carboy is about an inch and a half wide at the top.
If I was going to simply rack to another bucket, I'd keep it right where it was. But the reason I say using a bottling bucket as a bright tank is a bad idea isn't because of the headspace. Think about it. When you prime the batch, you have to stir in the dissolved priming sugar. If you do that, you will stir up and resuspend all the crud that settled out- defeating the whole purpose of racking the beer to begin with.
I'd keep it in the primary for three weeks, fine there if I was fining (I never have, so I'll defer to those who using finings), and then rack to the bottling bucket onto the dissolved priming solution when it was bottling time.