Construction costs are currently skyrocketing

Hot water when you don't need it and no hot water when you would need it for support in winter. ST is greatly overrated. Better to use photovoltaics and a water-to-water heat pump. To make it worthwhile in 10 years, you would need to save €1000 per year. That is completely unrealistic.

Next offer from an established company: 167€/sqm for insulation with rock wool (!) and not EPS.

Incredible what prices these are I come from the construction sector, normally it's about 90-100 euros / sqm Your companies must be making good money :-)

Crazy what prices
I come from the construction sector, normally it’s about 90-100 euros / m2
Your companies must be making good money :)

Normal is always relative. In Cologne, 2-3 years ago, 150/sqm with scaffolding and value-added tax was probably already common. Where do you get the service for 90-100? With everything included?

Normal is always relative. In Cologne, 2-3 years ago, 150/sqm with scaffolding and VAT was probably already common. Where do you get the service for 90-100? With everything included?

In the private sector or where? 2-3 years ago, in the commercial sector, we were at 70-80 euros net without scaffolding

Since I am a retiree in the private sector and gross

That makes much more sense than a photovoltaic system, since the hot water can be stored cost-effectively.

Wrong again.
In summer, solar thermal produces a huge surplus of hot water, and the storage tank just stands full in the corner. Pressure relief valves open, the system practically produces nothing anymore because there is no consumer. 300-1000l tanks then just stand around uselessly; nobody needs such amounts of hot water. Advantage of photovoltaics: surplus electricity can be fed into the grid for compensation, the system pays off significantly faster (or rather, reaches ROI within its lifespan).
In winter, both options come with few advantages. If solar thermal is supposed to contribute to heating, it becomes significantly more expensive due to the hydraulic complexity compared to pure domestic hot water support.

Solar thermal only exists(ed) in new buildings to make gas heating permit-compliant. Hence the two token collectors on many house roofs. Economically, it's nonsense.
It only becomes interesting with a particularly high hot water demand, e.g., in multi-family houses or if a pool is to be heated. In a normal single-family house, it’s best to steer clear of solar thermal as much as possible.