I also belong to the candidates where everything is sewn very close to the edge. I can confirm that the generally quoted prices here are significantly too high for some regions and it can also be done cheaper. In addition, some demands here for "normal middle-class people" are quite elevated, which one does not personally need and therefore comes off cheaper. BUT the warnings (by the way, at first I, like you, didn’t quite want to believe them, since sometimes the gut is stronger than the head) that small stuff also adds up, personal work may not be possible, or the buffer can disappear very quickly, are definitely true.
I was incredibly lucky that my injury happened only 2 weeks before the move. Had it occurred during a phase of personal work, I would have had to spend tens of thousands of euros that might have been missing later. Small stuff that couldn’t be anticipated beforehand amounted to almost 2k with us, even though I could borrow all tools for free. Had I had to buy or rent the tools for a fee, probably another 2k would have gone. Then there are 1-2 undisciplined things regarding the equipment, which fundamentally wouldn’t be necessary, and bam, money is gone again. It adds up quite a bit in the end. Then there’s a price increase and bam, more unexpected money gone. I can only recommend not to completely turn off common sense.
For example, I would also do without the garage. If there is money left over in the end, you can always retrofit it. Maybe also think about a floor plan that allows you to finish only the ground floor and the upper floor later (kind of like the parents of your friends). Basically a small bungalow floor plan with a pitched roof and an unfinished upper floor. Although this is frowned upon here by some, we did it this way. We have only been living in the house for 3 weeks, but so far we don’t miss the upper floor at all and are very happy. The only thing we already really miss is a shed.
We are also lucky that there should be enough money left over for a carport with a massive shed included. But I say lucky deliberately, because this was due to circumstances, e.g., the application for gas/electricity was submitted in 2018 and on 01/01/2019 a new price list was published by the provider, which would have made our connection almost 900 € cheaper. I only noticed this by chance and 900 € was waived from us as a gesture of goodwill, even though the invoice based on the old prices was already in place. We had 2-3 similar incidents, so in the end there is money left over. But these are happy exceptions; without them we would be more or less break-even, and a big problem could have broken our neck. Lucky break.
Result: you should believe the warnings of most users; unlike you, they have already been through the building process and do not rely only on hearsay from acquaintances/friends. At first, I also didn’t want to really believe it, but the longer the construction took, the more I realized that although things are often a bit exaggerated here, almost everything basically applies that was previously warned about.