The breville toaster ovens work fairly well. If you have it set to "6" and toast some bread, and put more bread in immediately afterwards, it correctly reduces the toasting time, since it is already warm.
I've had one of these for nearly a decade now, and I love it. I even use it in place of the regular oven when I've got something that needs baking that's small enough to fit, just because it takes about 1/4 the time to come up to temperature.
Since the bread is flat in there, it also makes it much easier to melt cheese on top of it without either getting the bread hot enough to melt it all by itself (which would surely leave it much browner than I like), or microwaving, which makes the bread soft and moist.
The main problem is toasters generally toast the bread for a set (adjustable) time. When you toast the first slices the toaster starts cold, but when you toast more slices right after the toaster starts hot so the slices get toasted more.
I think bigger problems is the moisture content of the bread itself.
Unless your bread is fresh out of the pack, depending on where you brought it from either refrigerator or rack, it's moisture content is going to vary changing your toasting time.
It's not even the price. I have a commercial toaster sitting in the garage, works well. But I don't have counter space or a need for a toaster with a conveyor belt. I'm just not making that much toast.
Is there a single toaster able to consistently toast bread to the same level each time? The technology must exist.