Is the offer for liquid screed and laying tack boards too expensive?

The basement is not suitable for anhydrite screed anyway. The basement is always a wet room in my case. Naturally, you always have more moisture there than on the upper floors. Anhydrite attracts moisture. Problems are pre-programmed there.

You have probably completed the drywall construction and the walls are newly plastered. Have you considered how much moisture you are bringing into your construction? You will never get it out again.

I estimate that with your flow screed, approximately 4000 liters of water are introduced. You know how much that is, where it distributes itself, and how long it actually needs to dry out completely.

If the walls were plastered as well and the masonry is still damp, it takes several months. No matter what any construction experts tell you. Not without reason are there always mold problems, especially in new buildings.

Since the basement is quite uneven and to ensure proper pipe coverage, the thickness ranges between 35 and 80 mm What alternative would I have?

Naturally, you always have more moisture there than on the upper floors.

What problems should there be with the thickness of the floor?

So far it was a utility basement, now it is being converted into a living basement.

How should the floor be leveled beforehand?