Basically I want the whole game to be in 2D but I want it to have structures with many floors and I don't want them to have a loading screen and for that I would need them to be in a 3D plane so as not to have many problems, as I would do to represent 2D scenes in a Plano3D without having many changes in the project?
how to make 2D nodes lie in the 3D plane
You could use Viewports, have each 2D scene in it's own viewport and then set the Viewport texture to the albedo texture of the plane. The Gui in 3D demo on the Godot demo repository does this, if I recall correctly.