Insulation of the top floor ceiling (cold attic)

This is not mainly about the financial aspect.

The idea was that I don't have to cut rafters etc., just distribute evenly and have a continuous clean top layer. Relatively simple, quick, and also cost-effective.

What speaks against putting concrete or screed on the Styrofoam?

So there is moisture in the concrete ceiling. Likewise, the moisture from the upper floor seeps upward through the concrete. There it meets an airtight space, condenses, and creates a breeding ground for mold. If you want the attic to be accessible, it either costs money or a lot of work. Cheap and good usually doesn't work.

if you come up every few weeks, always touch the [Unterspannbahn], there must never be any drops on it, everything has to be dry

how long would it approximately take until most of it is dried out?

Would concrete the top layer and then leave it open or put a dryer in when the roof is on so that everything dries through properly.

or possibly lay [Waschbetonplatten] on the Styrofoam.

The weak points are always the floor hatch or floor door, no super insulation and often only token seals

You cannot dry out a cold floor, it must be continuously ventilated