Zinspoker - Long term or short term?

The property is 500 sqm in size, location is Eifel.

When modernizing, take the [Denkmalschutzbehörde] into account. They will certainly want to have a say. There may be assistance (grants or similar) for this. However, it can significantly increase the cost.

Include the monument protection authority in the modernization.

Please no monument – that will be unaffordable (even if there are subsidies)...

The house is not a listed building

I think the topic "Energieeffizienz" is sufficiently covered and, if in doubt, payable by the OP.

The 10-year condition would also be great with 2.5%. This is (as of today) merely a money exchange for the bank without much profit.

That would be 80,500 kWh and almost 8,000 liters of oil

Oh dear, 8,000 liters of oil currently cost around €13,000.
This will increase by several thousand euros in the coming years due to the already decided CO2 tax alone.

Facade insulation, windows, front door.
Given the size of the house, I expect costs well into the six figures.

Me too.

Why not invest €200,000 and carry out a thorough energy renovation once and for all? It’s worth it.