Savings beginner with questions about the plausibility of the "rough" plan

That is not correct. Both allowances are always applied to the total income.

I have never claimed otherwise.

In your example, the second income would effectively pay 45% tax, no matter how high it is.

A reduction of taxable income (whether through allowances or actual expenses) always concerns the amount that is "cut off at the upper end." The marginal tax rate is decisive here.

If for married couples a tax rate of 45% applies from a taxable annual income of 120,000 euros, this means that at a taxable income of 130,000 euros the 10,000 difference would be taxed at 45%.
If, in a marriage, one partner’s income is already at 120,000 euros, then (mathematically) the high tax rate of 45% immediately applies to a second income because every additional euro pushes the total taxable income over the 120,000 threshold.
This is of course an extreme example, but it works the same way in principle with lower incomes and (marginal) tax rates.

It is therefore actually the case that in this scenario a second income—regardless of its amount—would be taxed at 45% from the first euro onward.

The tax classes in the annual consideration do not play a role either.

Again the question here: Where did you read that I see this differently?! It is always about the issue described here of the burden caused by the marginal tax rate. This circumstance becomes tangible when considering different tax classes (III/V).

As long as only one partner works in a marriage, they can use the tax allowance of the non-working partner.

There is only one allowance. For a married couple it is twice as high as for a single filer. It absolutely does not matter who earns how much or if one does not work at all.

If both earn the same and one gets a raise of 10k, it has the same effect as if one earns everything and the second takes a job for 10k.

Even if someone earns only 10k per year and chooses tax class 3 and the other earns 120k in class 5, it makes no difference at the end of the year, only the monthly deductions are different.

A second income therefore only actually brings more money into the household budget if it is significantly higher than the savings now being lost from the first income.

The assumption is incorrect and at most correct for the deductions. For that, there is 4/4 with factor.

There is only one exempt amount. It is twice as high for a married couple as for a single filer.

The point of dispute is now academic. I expressed myself this way to make it clear that the one joint exemption consists of the two individual ones and thus the one from the partner can be "used" in a way.

It absolutely doesn’t matter who earns how much or whether one does not work at all.

If both earn the same amount and one gets a raise of 10k, it has the same effect as if one earns everything and the second takes a job for 10k.

Even if someone only earns 10k a year and takes tax class 3 and the other earns 120k in class 5, it makes no difference at the end of the year, only the monthly installments are different.

I honestly don’t know how else to explain this.

Of course, in any case, having a second income ultimately leaves more net in the household budget. The only question is "how much" and, resulting from that, "is it worth it?" Due to a high tax burden on the additional income (regardless of whether it is a raise at A or B, or an entirely new job) and considering expenses related to the additional income (childcare, car, domestic help, etc.), in certain constellations (such as the OP’s here), there is not enough money left over at the end for it to be financially worthwhile for the couple (as an economic unit).

If the available income is 4500 euros, then after a payment of 1800 euros, 2700 euros remain (five euros into the phrase piggy bank). Many people, even with a house, get by with that.

Oh dear... Ashes on my head. When I looked at the planned costs in my table, I marked the payment - and then subtracted the 1800...

Therefore, I revise my statement and claim the opposite

Even though 1800 would personally be too much for me, it would also be possible within my house cost estimates, but with restrictions.

Hello everyone,

here is a little update...

Equity: approx. 40K
KFW: 50K
KFW Flexi: 80K
Bank: 390K
Total approx. 560K

This with a rate of 1800 and interest fixed for 30 years. Remaining debt 60K
This will be reduced with special repayments during the 30 years.

So, that is what is being offered to me. So far.

Everyone said that there is still some "room", since here and there something can still be done. (2nd decimal place).

Just so you know...