So, now I have some time and am sitting at the right PC, so that the text can be formatted at least somewhat properly.
An important point upfront: The following calculations are rough estimates pulled out of thin air. The given values are roughly rounded and do not replace a concrete, correct calculation with proper advisory programs from building society savings banks and/or banks or brokers. Rather, they are meant to aid understanding and give a rough direction of what I consider reasonable. This is also only my opinion and not the one correct and best solution.
First a note on 100,000 euro Riester. Most contracts require 40% savings amount. With a maximum subsidy amount including allowances of 2,100 euros p.a., after the mentioned 13 years this is about 27,300 euros, neglecting interest and compound interest, as well as neglecting closing fees. So at most 30% is saved. Does it work like this?
Now my proposal:
He:
50,000 gross income
4% correspond to 2,000 euros, less basic allowance = 1,846 euros p.a., means 155 euros monthly
With 15 years savings period = 30,000 (40%), means 75,000 euro home Riester.
She:
18,000 gross income
4% correspond to 720 euros, less basic and 1 child allowance = 266 euros p.a., means 25 euros monthly.
With 15 years savings period = 10,800 (40%), means 27,000 euro home Riester.
In total, that is 102,000 euro home Riester, with 15 years pre-financing at 2.6% p.a. without repayment that is 221 euros monthly interest.
A further 98,000 euros must be financed. For example, annuity loan 15 years as full repayment!, also at 2.6 % p.a. (5.7% repayment) is 660 euros monthly and this loan is paid off after 15 years. (alternatively shorter fixed interest period and/or repayment as needed, reduces the loan rate but increases the risk)
Total: 1061 euros monthly (+KFW, since conditions are almost the same everywhere)
After 15 years, therefore, "only" the repayment of the Riester loan according to the applicable conditions there remains.
This calculation takes into account, in the Riester component, the "optimally highest proportional subsidy without unnecessarily inflating this component and eliminates the interest risk. However, it is also the most expensive option monthly. But even if you choose the annuity loan with a residual debt (lower rate), it should still be manageable from a risk perspective.
The interest rates are chosen as examples and may differ, especially regarding the Riester pre-financing.