Not wanting to be stubborn, but the architect has already been annoyed that we question everything so much. Of course, hindsight is always 20/20, but for us, this is simply the first time.
Now you slowly know how things run in construction. If you’re not careful, you quickly pay twice as much as planned. So I can only advise you to take back the responsibility, give the architect a tired smile yourself for once, and turn off the money tap for now. The project is on hold until there is a concept where all costs fit logically and transparently into the budget and the offers are also available. Otherwise, goodbye gentlemen.
You should do the budget planning completely yourself. That belongs to it if you are not Bill Gates. Then you also discuss with those involved on a different level when you know what you’re talking about.
Most people here have a long Excel sheet on their computer listing every tiny item. We had 3 columns: Planning (material, hourly wage, possibly offers, etc.), Offers (preferably always 3), and Invoice total. The goal, of course, was for the columns to be close to each other and the total at the bottom to stick to the budget. You spend about a week on it and then diligently correct it throughout the entire construction. This way you keep an overview and a certain level of security. I would never rely on external companies here, especially not if their salary rises when the total amount gets higher.