The rate for the shell construction is, as already explained, much too high. I just don't understand the discussions about the final payment.. 1%, 3%, 5% completely irrelevant if it is not specified which services correspond to it.
At the final acceptance, defects that have not yet been discovered will most likely not be discovered either.
Much better is an inspection with an expert at relevant intermediate steps (e.g. base slab before pouring, roof structure, etc...) Here defects can already be indicated and withholdings made if necessary.
This is at least as important as a balanced payment plan.