Turnkey vs. Individual Contracting

I would say: tax on everything you just buy from the mentioned property developer

That's right. So

EVERYTHING means
1) tax on (land + shell house) or 2) tax on (land + shell house + sum of the prices of the individual trades)

1) if only the shell house with the land is purchased and 2) if construction packages are also purchased - caution: two invoices or leaving time in between is/are useless.

Being from the village doesn’t mean the craftsmen are ready at a moment’s notice or good.
I know two people here who wanted to do it like you. Construction start for both was spring 2017. Move-in?
6 months construction halt because of windows. 3 months because of stove fitter, electrician, etc.
One almost grudgingly carried out almost everything himself with his brothers. One brother meanwhile built a ready-to-move-in house next door.
The other one said at some point no matter the cost, the main thing is that a craftsman comes. Now, he’s been building for a long time due to defect repairs. Which, of course, there weren’t any.

I’ve been cleaning door handles since February so that a heating engineer will replace leaking pipes and install new heating registers in the workshop. We know each other, everyone assures me the heating will work by autumn. I watch September pass and the heating doesn’t run. Totally - they can’t.

Another example
In October 17 the roofer dropped out on us. Sealing new garage to old retaining wall. In September 2019 it was finally completed. First searched for months. Checked all acquaintances. The company that did the earthworks and concrete works for us then found someone (good reputation as builders). Unfortunately, the roofer was then sick for months.

I am not a professional, that is correct. You know people here and there, you know the heating engineer, the tiler is an acquaintance. An advantage I still have, hope that is significant, my father built his house himself about 20 years ago.

I also know many! The butcher, the baker, the guy from the public order office. I have never gotten the sausage, rolls or bulbs at a discount! Building a house and the trades have to be coordinated. It also gets tricky with warranty. When something doesn’t work, the trades start passing the buck. A house costs money, the general contractor also wants to get paid for his services. I do not accept the general statement “you can save a lot of money there.”

My father built his house himself about 20 years ago.

....what does that mean to you?
If he really did everything himself, I claim that it might not measure up to some other houses. I certainly don’t want to offend anyone, but an all-rounder cannot deliver the same high quality in all the different trades.
The question is which standard you want to achieve and honestly, I would just ignore such pub talk among friends.

As a layperson, one really has no idea how the individual trades interlock. It doesn’t happen in a serial manner either; rather, trade A does a part, then trade B has to step in, then A can continue, and only then C... you have to know this, otherwise the already mentioned chaos will occur. It is also important to coordinate the "handover points" from A to B to A to C.
I once came through the door where the site manager and craftsmen were negotiating:
“Then trade X will have a problem if we do it this way.”
“Don’t let the client hear that!”
“She’s right behind you...”
Awkward silence...
X was much later in the schedule; at that time, it wasn’t even clear which company it would be... and here you have another example of what can go wrong. Here is a non-optimal execution and the problem only emerges later in a subsequent trade.
It is also quite an art to think of absolutely everything – some things are very obvious (you need windows, and heating, and electricity, and a toilet), whereas others are rather hidden in the end but still have to be done.
My opinion: taking coordination into your own hands is only for people with expertise or strong nerves, plenty of time, and excess money. Everyone else should please hire a site manager (e.g. an engineering firm).

And PS: for me, the interior finishing cost about €1000/m² in standard quality, just as a reference and related to the initial question whether €190k is okay for that. (2019)