Turnkey vs. Individual Contracting

As a layperson, you really have no idea how the individual trades interlock. It doesn’t work serially either, but trade A does a part, then B has to step in, then A can continue, then finally C... you have to know that, otherwise the already mentioned chaos occurs. It is also important to coordinate the "handover points" from A to B to A to C.
I once came into a door where the site manager and a craftsman were negotiating with each other:
"Then trade X has the problem if we do it that way."
"Better not let the client hear that!"
“She is right behind you...”
Awkward silence...
X was much later in the schedule, at that point 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 arises eventually with a subsequent trade.
It's also quite an art to think of absolutely everything – some things are very obvious (you need windows, and heating, and electricity, and a toilet), others remain rather hidden in the end, but still need to be done.
My opinion: Taking on coordination yourself is only something for people with know-how or strong nerves, plenty of time, and surplus money. Everyone else should please commission a site manager (e.g., an engineering office).

And PS: for me, the interior finishing cost about €1000/m² in a normal standard, just as a rough guideline and regarding the initial question whether €190k was okay for that. (2019)

In addition, each trade likes to shift the responsibility onto the other, and as a result, you are always stuck in the middle having to make a decision whose consequences you cannot necessarily foresee.

house construction and the trades must be coordinated. It also gets tricky with warranties. When something doesn’t work, the trades start blaming each other.

That's how it looks. We built with a general contractor, few own contributions and some self-contracting. The interface coordination should not be underestimated.
If one of the parties slacks off (executing craftsman, site manager, client, other involved trades) or doesn't feel like coordinating, it gets difficult, meaning: protracted or expensive because "I thought that was already prepared by x or will be done later by y."

If something goes wrong (for example, with us, water-damaged insulation between joists or cracked screed), I was infinitely grateful that no trades outside the GC’s scope were involved. So I only had my site manager as a contact person, who developed solutions with the involved trades according to instructions – I didn’t have to deal with annoying questions of blame or financing. Silver lining in misfortune.

Moreover, the word of the respected site manager from the GC carries much more weight with the individual trades due to significantly larger overall contracts than if I, as the client, addressed the same thing directly.

Whether individual contracting is really cheaper in the end cannot be generally said. The more individual you build, the more lucrative individual contracting is in my opinion.

We contracted individually and organized the site supervision as follows:

[*]Contracted earthworks and civil engineering up to the foundation slab and hillside support including piping for utility connections to the street to a local company. We were repeatedly on site, took care of the workers, praised them, thanked them, and let them explain what was happening. Cost drivers were proportions of soil class 7 and hillside location.
[*]All other also local companies were selected together with the carpentry company that built the house. (In your case, that would be the shell builder). They included 100 extra hours for construction coordination (scheduling, coordination of trades) in their offer. We were involved as mentioned above and agreed on detailed executions with the individual companies (materials, equipment, colors, things we came up with, proposals made by the companies which we accepted or rejected). Cost drivers were our detailed decisions that went beyond the original scope (clay plaster with straw content, larch wood cladding of the house, special wooden flooring, special tiles, additional outdoor water points, Italian glass mosaic, special lighting, switch program...)
[*]We supplied some materials ourselves because they were not available from the companies. Overall, this was still attractive for the craftsmen across the whole project.
[*]We accepted higher hourly rates because you can’t get the best quality if you only use the cheapest labor.
[*]Every company can use our house as a reference object for further customers. Since the house caused a stir and was a talking point in the region due to its location and appearance during construction, this was an incentive for everyone involved from the outset.

The construction phase was a very happy time in our lives and largely free of stress. All in all, considering the quality, individuality and flexibility during construction, we now have a house which we certainly would not have gotten from a turnkey provider – and certainly not for the price.