Construction costs are currently skyrocketing

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.

If even the big players are now pulling the emergency brake, the average citizen should also better keep their hands off it. Anyone starting a new build now needs good nerves and a 30% reserve. Ironically, however, this has something good for existing properties. If new builds crash spectacularly, then the prices of all existing houses (whether 4 or 40 years old) will continue to rise. Because then a still small supply meets the buyers, who then renovate a house instead of building one themselves. Because hardly anyone from the middle class will be able to afford a new build so quickly. I’m saying it: existing properties and new builds are closely interconnected. If the price goes up on one side, it rises on the other half as well. Because what else does a collapsing new build lead to? Increasing rental prices, because hardly anyone can afford to build anymore. And Vonovia just can’t get the 20€ per square meter in every village. Even 12€ would be sporty from my point of view.

On the other hand ... the prices for unsanierte 70er Jahre Häuschen are still beyond good and evil. However, they still seem to make up the majority of the offers.

Yes, the prices will still go down by -20%. But that is still more expensive. Because the prices are still at a pre-Corona level from 2019. And paying these prices with high interest rates is already madness. But what else can one do? Sometimes I also cannot understand the municipalities. For years there have been discussions about the expansion of a construction area, which is now supposed to start in the middle of next year. Much too late for almost everyone. At these prices. And even though construction was supposed to start there already 3 years ago. At such a pace, it is no wonder that existing properties remain expensive.