What conditions are possible? Capital investment; 6 apartments

Well, 12,000 + 2 * 1,500 also equals 15,000 – so one more vote for the tax advisor before unconsidered actions.

Thanks for your answers! But isn't it basically sensible to repay quickly? Once the thing is paid off, we'll just look for something new

Not necessarily.
You can also just repay the minimum necessary and save the surplus as new equity for the next purchase.
The advantage:
1. You have the tax savings and
2. The next property can be acquired much sooner and can thus generate returns again.
For this to work, the borrowed capital must have a lower interest rate than the overall yield (return) of the property.
Check online for Leverage Effect or also Interest Rate Lever.

Yes, you just end up paying the bank interest for longer instead of paying the government some kind of taxes. Everyone can do as they please - I prefer to repay my outstanding debts quickly and then pay taxes on the rental income as usual. At least then I have access to the rental contract (after taxes), instead of endlessly paying bank interest just to save on taxes.

Sometimes I think people occasionally confuse the requirements between private individuals and companies. :)

Well, 12,000 + 2 * 1,500 is also 15,000 - so one more vote for the tax advisor before rash actions.

So the 15,000 € were actually only meant to make it clear that we can afford the 6,500 € installments including rental income (about 2,000 € / month) of the new property. The actual salaries are higher. With my father you are roughly right, my brother has been earning his living as a doctor in Switzerland for a year (hardly any taxes) and I, as a civil engineer in a management position, already pay good taxes. My brother and I are still in tax class 1 for income tax purposes and are well above the 1,500 € net mentioned here :P

The other house has only been debt-free for 2 months. So we will only enjoy having to pay taxes on it in about 1.5 years. However, I am fundamentally one of the few people on the planet who gladly pay taxes. It would be much worse if I did not have to pay taxes when I earned little money.

So the €15,000 was actually only meant to make it clear that we can afford the €6,500 installments including rental income (approx. €2,000 / month) of the new property.

My main point was actually to make clear that the distribution of income is also relevant, because the person who pays the most taxes can logically also deduct the most. According to your information, the brother is therefore already out, since he does not pay any taxes in Germany, if I understand correctly. So once again for our high earners: Pay attention to the details *before* you sign anything.