If you really want to build with the company, look for 2-3 core points that bother you. See if you can enforce adjustments and evaluate your risk at the end.
that is certainly the right approach. the fixed price valid indefinitely is really very very important. otherwise, they could create the wrong building applications, it gets delayed (from their point of view of course not their fault but the fault of the office) and suddenly it becomes unpredictably more expensive. 1 year is actually quite little. it should actually be negotiable without major problems, as it does not involve any legal aspects or similar.
the first point is already more specific. if you have a development plan, any architect can submit the building application so that it is also approved. it is not a matter of luck at the office if all requirements are met. and furthermore, the company would then also have costs and naturally does not want to bear the risk. i consider it pretty hopeless to get that.
important is also a construction time guarantee, which is provided with a reasonable phrase. you can gladly post this one. in what price range does the construction company move for comparison. a "rote buchstaben" does not say everyone something. Viebrockhaus? you can name the construction companies here if you don’t immediately add an insult.
the fact that a company is not willing to fiddle around with the contract much also shows that the company is probably doing well and they have enough orders. what that means you have to decide for yourself.