Use Riester and family support in a targeted manner

Child benefit is not subject to the progression clause, so it does not increase the taxable income. Parental allowance, on the other hand, does.
So if you have no other taxable income, then the parental allowance must not exceed €8,000 in order to remain under €80,000.

Child benefit is not subject to the progression clause, so it does not increase the taxable income.
Parental allowance, on the other hand, does.

No, that is incorrect. Neither child benefit nor parental allowance increases the taxable income. The progression clause only applies the higher tax rate to the taxable income.

Regardless of whether the €80,000 gross is undercut or not, the question remains whether this funding program is even of any use to you. It is merely a funding program that offers lower-interest loans to virtually simulate equity. But these loans still have to be repaid. Furthermore, I couldn’t quickly find any information on the maximum amount for these loans and the discounted interest rate. Assuming a loan of €150K that one might get through KfW at maybe 2% below market interest rates, there would still be €450K left at market interest rates, and both have to be paid back. So, I don’t know if that can work if nothing has been saved so far and you also have to support 2 more children.

I wouldn’t fixate too much on the funding; the details are still missing. What does interest-subsidized mean in practice? How much savings are you specifically hoping for? 3% for the funded loan with a 4% market interest rate is still subsidized. On average, however, that still results in a payment of around €2500. Even with a 2% subsidized interest rate, you won’t be able to save the financing. I’m sorry to have to put it that way, but these are just small amounts in the overall context. Only EH40 is supposed to be subsidized, if I understood correctly. You recoup the interest savings through the surcharge. The financing needs to be structured so that it works reasonably well without subsidies. Anything else is financial harakiri.

That is why my question was to what extent Wohnriester can be used effectively to achieve a combination of the various funding options without equity, since Wohnriester grants a high allowance. I secured [zuhausedirekt15 2019].

Let's be clear: what's actually included in Wohnriester? With those amounts, 10k more or less doesn't really make a difference. And if you haven't managed to generate a monthly surplus so far, you won't have any reserves to cover potential additional costs. You have so far dodged the question of where all the money is going and how you plan to manage with 2 more children.