This.I have no duck experience, but I do have chicken experience.
If you have a BIG brooder, you can have a heat lamp keeping one corner warm enough for newly-hatched ones, and the area further away will be a bit cooler, and the furthest half of the brooder is at room temperature (no added heat at all.) Then there is no need to adjust temperature at all-- the chicks run in and out of the warm areas and cool areas, but generally spend more time in the cooler areas as they grow. I know this works fine for raising chicks that hatch over a period of two weeks or so.
I assume something similar would work with ducklings too, but I do not know how quickly they get how big-- you would need to be careful that the big ones cannot trample the little ones.
Note, my idea of a big brooder is 4 feet by 6 or 8 feet. I'm not sure how small you can get it and still have enough cool space. Bigger yet is no problem.
My brooder is only 3' x 6' but it is in the coop. I put chicks in there straight from the incubator, whether the outside temperature is below freezing or we are having a heat wave. One end stays toasty warm, the other cools off to ambient. In winter I might have ice in the far end so the chicks stay on the toasty side.
I don't know how many ducklings you will wind up with but that looks really small. They grow fast. Having a second brooder may come in real handy for more than just size differences, but the absolute easiest way to never have to worry if your brooder is too warm or too cold is to make it big enough that you can heat one end and let the other cool off. The babies will go where they are comfortable.