Construction costs are currently skyrocketing

Huh? At the well-known photovoltaic shop with 4 in the name, for example, Hyundai 410W Fullblack is currently in stock for €212, and that's rather expensive. Without the extra frills, it's usually more like €160-170 from Ja Solar and others. Modules are not the problem at all. A well-known industry site just reported that a significant price drop is expected this year. What's still difficult to get are inverters. If you don't want to pay weird prices or wait 9 months for delivery, it will just say Huawei instead of SMA or Fronius...

250€ for a 405Wp is already very expensive if you don’t buy it from the solar installer. 150€ for 400Wp is about the average price for glass-foil, with negotiated offers you can also pay 130€ or less for a corresponding quantity purchase (e.g. 1-2 pallets [36-72 pcs.])

The solar technician in general is a sort of botcher.

There is no training profession or master title called solar technician. These are usually some sort of life artists. If I call myself a solar technician tomorrow morning, then I am one. So who exactly is the solar technician? The electrician, the roofer, the scaffolder, or maybe even the bricklayer?

Such fields of activity are always nice where you can practically demand 100 euros per hour without any qualification. That is especially fun when you find enough fools who then pay such prices.

If I buy a used scaffold for 2000 euros and can then charge the customer 1000 euros for 2 days, then it is a good deal. If I buy 1 kWp of modules in wholesale for 400 euros, then also. Another 120 euros for the inverter and 100 euros for small mounting materials. So material costs of maybe 700 euros make a profit of 800 euros per kWp.

So an extremely profitable business.

The solar installer in general is a kind of bungler.

There is neither a vocational training profession nor a master title called solar installer.

True.
But only true because you forgot an r:
"The title 'Solarteur' is registered with the European Patent Office and protected as a word mark throughout Europe."

To work as a solar installer, you must first complete training in another technical or manual trade such as sanitary-heating-air conditioning, electrical, construction or finishing, as well as roofing or facade construction, and complete it with the master craftsman title. Alternatively, it is also possible to undergo further training as a specialist in solar technology as a skilled craftsman with professional experience in the solar field. But engineers can also benefit from this specialization. It is therefore an advanced or continuing training for skilled manual workers who want to specialize in the solar field.
The training lasts four months if completed full-time. Those who pursue further training as a solar installer alongside their job need about six to eight months. At the end, participants take both a practical and a theoretical exam.

I want to have your modules. With 405 kW per 250€ module, I would compete with the municipal utilities.

Mine cost more. I pay a total of 15k for everything. That is 25 modules, including a 10 kW SMA inverter and installation with materials.

News on construction costs/construction planning for 2023…

The company Vonovia is pulling the emergency brake! "We will not start any new construction projects this year," said development board member Daniel Riedl to the "Westdeutsche Allgemeine Zeitung."
Reason: "Inflation and interest rates have risen enormously and we cannot close our eyes to that." A "significant number of construction starts," among others in Berlin and Dresden, have therefore been postponed, Riedl said. Most developers are acting similarly.
The Vonovia board explained: "For properties that we could previously offer for twelve euros cold rent per square meter, we would now have to go more towards 20 euros to cover our costs of 5000 euros per square meter." These rents are "completely unrealistic" in large parts of Germany.
That means: According to Vonovia, a 70-square-meter apartment would have to be offered for just under 1200 euros COLD rent.