Cistern - flat tank or round tank - groundwater

And it exclusively concerns the part below the 1.9 meter depth

For me (4m³ round tank) that would be about 50 cm. and the top 80 cm is the shaft, so the rest is also covered with earth. I wouldn’t worry about that either. It weighs at least 120 kg empty.

I do not only read 1.9m in the report but also that you often have to reckon with 1.5m and under extreme conditions even up to 0.5m. I would not align the entire plan to the extreme value now, but planning with 1.9m as the maximum is definitely wrong. Presumably, the cistern will also be filled under extreme conditions, which has a relieving effect (or actually more of a burden?).

We have similar conditions.

I have read about "Grundwasserventilen" that are installed in the tank beforehand. They allow water to flow through a valve in only one direction.

Maybe that is what you are looking for.

Specifically, a 5000-liter tank is to be installed. It is then either 2.5m high plus 50cm deep, so 3.0m excavation. Alternatively, a flat tank with a height of 1.5m and an excavation of 2.0m. Could the 3.0m excavation be a problem with the groundwater level?

Water will very likely flow into the pit. To determine when it becomes critical and to what extent it needs to be secured, you should ask the [Tiefbauer]. For DIY work with a rented mini excavator, that would be too risky for me.

Good cisterns are, in my opinion, also approved for installation in groundwater layers made of plastic (depending on the manufacturer and tank geometry up to the tank shoulder).