Construction company halts work despite overpayment

The company's weather excuse is nonsense,

In practice that may be true, for example you can still work with adhesive mortar just above freezing point (around 1-2 degrees), but according to the manufacturer further measures (see winter construction site, if commissioned) must be taken from 5 degrees onwards. ...but no specialist lawyer will refer to practice. I myself had a winter construction site at the time when the OP was still being built. The temperatures were really borderline at that time. 1-2 degrees less and it wouldn't have gone any further for weeks with me.

A lawyer specializing in construction or contract law and the strategy of reaching a settlement seem unsuitable to me here. I see it as urgently necessary to b) also consider the matter from a criminal law perspective and a) to thwart the GU’s attempts to buy time. Regarding litigation tactics, I recommend you obtain a second opinion from an insolvency lawyer. It seems to me that the offense of down payment fraud is fulfilled here: each payment for a construction or performance phase is a legally independent act, i.e., if someone accepts a payment "cash on delivery" for the eighth phase of a project and fails to provide the service in return, then that is fraud, even if they have provided services in the seven previous phases. In the case of “Antragsdelikte,” no criminal investigation will be initiated without your complaint – the civil proceedings under construction contract law do not yet cause the prosecutor to take interest in the matter.

With a company that only employs subs and/or demands 40,000 upfront, no one with any reasonable life experience would actually want to build. But that's how it is nowadays, everyone really thinks they can play with the big players. And then they even complain about how bad the world is.

No one with at least some life experience would actually build with a company that only employs subs and/or demands 40,000 in advance.

That's nonsense.
5 or 10% results from preliminary planning, building application, etc. A layman is not involved in that. That it then amounts to 40,000€ when the total is 400,000€ is just how it is. A general contractor will have his payment schedule notarized.
And: a general contractor has his subs since he coordinates himself and does not do the work himself.

What nonsense is written here is unbelievable. I would strongly recommend some to hold back a bit here, because the OP has enough trouble and unfounded know-it-all behavior does not help.

My goodness!

: We withheld €7490 by mutual agreement (defects: basement 10 cm too far out, boundary stone broken, wastewater pipes and light wells not in sand but in construction rubble, wrong backfill material of the excavation pit, etc.) The fact that the basement now protrudes 10 cm too far looks bad and does not fit into the landscaping plan. There should have been 2x2 car parking spaces in front of the house and the light wells integrated into them, so to speak drivable. The platform was supposed to be stroller-friendly, now we will have 2 steps down and 1 step up. The withholding was completely justified, since since 1.1.2018 a security in the amount of 5% of the gross contract sum is legally required according to § 650 m paragraph 2 of the Building Code, either in the form of a guarantee or withholding until the 5% is reached. Since we did not receive a guarantee from the contractor, this would have been about €24,000 for us, which we have not even nearly exhausted. Regarding the statics: We concluded a construction contract with a guaranteed fixed lump sum price. The planning was done by the contractor; there seems to be no execution planning at all. If the contractor suddenly realizes after completion of the shell of the ground floor that the ground floor should have been built differently to support the load of the roof structure and then has to build walls differently in the attic because no execution planning was done, are we supposed to pay for that? I think many do that to avoid stress, and the contractor probably counted on that. But I am sure that even if we had paid this item, a lot more would have come up.