Is 30cm of ThermoPlan S9 brick sufficient?

The 36 cm brick will probably have a U value of about 0.24 W/m2K instead of 0.28. Accordingly, about 0.04 * 70 kWh more heating per year per m2 of wall. With 100 m2 of wall, that's 300-400 kWh of heat per year, about 100 kWh of electricity. Now you can compare that to the 8,000 € and calculate whether your grandchildren will live long enough to make it worthwhile. Or you install your photovoltaic system for 8,000 € / 5 kWp on the roof and have 5,000 kWh of electricity generation every year.

The heating will not be more expensive because of this; it is almost always oversized anyway, and the flow temperature and annual performance factor can be influenced much more by the design and heating circuit planning. Use a lot of insulation where it is inexpensive. So in the roof and as free of thermal bridges as possible (that actually requires some brainwork).

This is not rocket science, and even though the numbers are only rough estimates, it's a "no-brainer."

That is quite a meaningful rule of thumb calculation - thank you!

Still for my understanding:

[*]What do the 70 kWh refer to?
[*]The conversion from the 300 - 400 kWh heat to electricity is based on the annual performance factor, right?

Thank you also for your other tips!

That is quite a meaningful rule-of-thumb calculation - thanks!

Just for my understanding:

[*]What do the 70 kWh refer to?
[*]The conversion from the 300 - 400 kWh heat to electricity is due to the annual performance factor, right?

Thanks also for your other tips!

The 70 kWh per m2 is a guideline value for the annual heat energy saved when the U-value of a component is improved by 1 W/m2K.
Yes, the conversion is based on the annual performance factor.

and the flow temperature and the annual performance factor can be influenced much more by the design and heating circuit planning

That's right. The combination of well-planned heating surfaces and a well-designed and operated heat pump brings much more than marginally better insulation. Even the ventilation system contributes more to heating demand and the annual performance factor than the best wall could.

Your time is better spent on heating system planning.

Add photovoltaics as long as it's still worthwhile, and you will have a significantly lower energy consumption than many better-insulated new buildings.

Only the 36 cm bricks could be interesting purely from a soundproofing perspective. More mass is always better here. For this, one can well look at and compare the sound insulation values. 5-10 dB is perceived roughly as twice as loud. The windows must not be excluded from consideration; the best wall is useless if the windows are noisy...

Generally, I wouldn't necessarily heat with a heat pump in a house built to the energy-saving ordinance standard

It's a mindset. 3-4 years ago, that still corresponded to KFW 70. People talked about it just like they do today about the current KFW 55 — fancy stuff that wouldn’t really be worth it, although it really depends on that and also on future energy prices. The cheapest kWh is the one you don’t need at all. Grandma in a 60s building wouldn’t have thought back then that she’d have to spend 250€ a month on heating oil. But those are timeframes you simply can’t speculate about.

Monolithic is rather expensive as an upgrade; if you’re building an ETICS anyway, you just add 2-4 cm and have basically done the essential part.