Hello Fabian,
a little feedback based on my current experience.
We had our house planned and approved by an architect plus LV6 ([Leistungsbeschreibung für das Ausschreiben der Gewerke]).
Originally, we planned to carry out the project with a general contractor (GU) based on the specifications and construction plans but quickly found out that the prices quoted were sometimes outrageous. There were also price differences of up to €200k.
Since I am fortunate to know some acquaintances and friends in the construction industry, we have now switched to carrying out the project via individual contracts, limiting the number of trades to coordinate by bundling through one company. For example, demolition, shell construction, and roof are done by one company, as well as the interior finishing. Windows, facade, heating, sanitary etc. are done via individual contracts.
The advantage for me is that I am now able to make changes even during the construction phase (okay, costs always money), and could get significantly better prices.
The disadvantage is definitely the considerably increased workload due to tendering, coordination (despite having a construction manager), as well as the expected longer construction time.
Surely there are more advantages and disadvantages, but basically you have to weigh for yourself what is best for you. Until now, despite the stress for my wife and me, this has been the right way.
Or rather, an expert should always be mandatory. I am not a professional and cannot immediately see defects.