Energy saving through better stones

For €120,000 there is an €18,000 repayment grant

Yes, that is not little, and if the stones were the deciding factor. And you are definitely better positioned for the future!

If better insulation is achieved through mass (=filling?), it also provides better sound insulation. However, this cannot of course be factored into the same economic calculation.

Soundproofing is achieved through mass, not through better thermal insulation. The differences there should be negligible.

Achieving funding levels is particularly attractive recently. KFW 18-20k, BAFA also already 4-5k last year for heat pumps.

Better insulation leads to lower heating load. For example, if you use a brine heat pump, it can be enough for a "step down" to a smaller and therefore more cost-effective heat pump investment. Eventually, a replacement for the heat generator will also be needed, which further amplifies the effect.
The drilling also costs per meter = extraction capacity, which can quickly save a thousand euros.

One can also hope to do something for the resale value.

The overall concept is decisive.

Some arguments have already been mentioned as to why it could still be worth it.
- KFW !!!!
- Resale value
- Smaller heat pump
- Possibly less floor heating pipe
- etc.

I'll throw in another point.
Your financial loss calculated over 40 years is €1230. That's laughable! For me, that alone would be worth carrying the additional costs for my ecological conscience.

And finally, I can tell you that your calculation is probably not correct. I would assume an annual electricity price increase of 1 - 1.5%. That makes the situation look even better.

So from my perspective, in this case, a clear recommendation: The stone with the lower U-value pays off!

Regards
Specki