In my opinion, arrogant is the behavior of the home builder, namely no longer responding to the customer. I believe one must free oneself from the misconception that the home builder wants to achieve maximum customer satisfaction; rather, he only thinks about his own pocket. The proposal from construction experts to pursue a legal evidence procedure involves considerable costs. This means filing a lawsuit in court, advancing the court fees, and then commissioning an expert appointed by the court, whose costs must also be advanced first. In addition, there are the costs for one's own lawyer and, in case of losing, the costs for the home builder's lawyer. A few thousand euros that must be advanced quickly add up; with a dispute value of, say, €25,000, we are easily talking about €10,000 that must be advanced first.
The second tip, to use the withheld money for defect rectification, is quite a risky undertaking. In principle, the home builder has a right to remedy defects, i.e., one must grant him the opportunity—also several times—to fix the defects. If one interferes with this right and thwarts this right to remedy defects through careless self-rectification, the home builder will rightly demand the outstanding and withheld sum, using a court, with the usual additional costs and an interest of 5% above the base rate on the claimed sum.
I think, in principle, one will not get around having a lawyer since one can very quickly get onto thin legal ice, which can also become very expensive very quickly in the case of construction defects.