In principle, you are right that it is possible to keep an object in the air without expending any energy. Think about a blimp, you can fill it with helium, and then just let it float. It doesn't have to do anything to float, it just floats because it is lighter than air at ground level (the air gets less dense as you go up).

In fact, it is similar to a boat floating on water. That seems like a more intuitive example of how something can float without expending energy.

Now your drone is probably heavier than air, and so it will expend energy, but that is ok. You have shown that it is possible to float without expending energy, not will never expend energy if you float.

In principle, you are right that it is possible to keep an object in the air without expending any energy. Think about a blimp, you can fill it with helium, and then just let it float. It doesn't have to do anything to float, it just floats because it is lighter than air at ground level (the air gets less dense as you go up).

You can even make heavier than air objects float without expending energy. One way to do this is to make a diamagnetic material levitate with magnetic fields. The classic demonstration of this is the levitating frog. You should be able to find videos on youtube.

Now your drone will expend energy, but that is ok. You have shown that it is possible to float without expending energy, not will never expend energy if you float.