Construction contracts - how reliable are they?

How is the "Flexpreisvariante" to be understood and why should one prefer this variant over the alternatively offered Festpreisvariante? I don't understand!

We decided on the flexible price option because we were led to believe that it is still a fixed price, but that we would be credited with discounts or prices below the calculation, so to speak, and that this might provide more money available for other trades.

We decided on the flexible price option because we were led to believe that it is still a fixed price, but we would be credited for prompt payment discounts or prices that are below the calculation, so that we might have more money available for other trades.

and how did it turn out?

Since we never received an exact plan regarding the calculated costs, I was never able to verify whether the prices for the individual trades were calculated correctly, more expensive, or cheaper... Ultimately it doesn't matter, since the house was completely miscalculated, the construction will be at least €100,000 more expensive than planned. Since we did not choose the fixed-price option, we now have to finance more. Argus company terminated the contract after I stopped my payments and initially sought to enforce through a lawyer that the defects incurred up to that point be remedied.

Currently, we are aiming for a lawsuit. According to the expert report, we paid the company €40,000 too much. We currently do not know whether we can continue the construction.

because we were led to believe that it was still a fixed price

You signed a construction contract, so there must be something concrete about the flexible price in it? You hoped the price would only be flexible in one direction (namely downwards)? Sorry, you can't seriously sign something like that. Then simply not paying anymore and thereby provoking termination by the construction company is also not a solution.

Okay, the construction company offers a (in my opinion) dubious pricing model, but you yourselves signed this nonsense, you can't blame anyone else now. Your "flexible price" was certainly much cheaper than comparable offers. I wonder why.

I do not want to blame anyone because we signed. I also did not just "stop paying," but requested the company to first fix the significant defects (which an expert has identified). These are not trivial matters when walls are crooked, the concrete staircase is floating in the air because it was installed incorrectly, and so on... It is normal to withhold payment if a trade is not executed without defects. Or do you pay full price for faulty goods? Yes, we were naive when we chose the flexible price option. I trusted the contract as I had it reviewed beforehand by a lawyer specializing in contract law. I only want to warn against doing the same and am looking for others who may have been affected!