Although if I interpret the fluid rules right, the lake would drain pretty quick even through an 1x1 drain, as the water would just teleport through, once the drain is completely filled with water.
This is correct. The rate of drainage has absolutely nothing to do with the size of the hole. The only constraint on the drainage rate is getting rid of the water at the far end of the drain. Dumping it into an aquifer is a quick way to do that, but if you don't have one, you could just let it go into a cavern layer. Even if the cavern totally fills, there should be enough map edges down there to drain the water faster than the lake map edges let more water in.
That is the most practical way to drain a large body of water and it's how I'd do it. But for the sake of novelty you might also consider dropping a fire bin. You need to set up a magma-safe bin (e.g. iron) with a long-burning object inside of it (anything made out of lignite, bituminous coal, or graphite), expose the bin to magma so that the heat ignites the object, and then drop the bin (but not the magma) into the lowest point of your lake. The bin will keep water from contacting the burning object and extinguishing the fire, so the heat will just evaporate all of the water.
Or at least that's how it worked in 40d. I haven't tried it for a while.
