We are not making progress in financing

Not for me. Unfortunately, you didn’t say what credit interest rate the building society saver has and with what rate the building society saver is then serviced. That’s why it’s difficult to calculate exactly. If I calculate the whole thing only with annuity loans without a building society saver, I have a remaining debt of under €72,000 after 15 years with a rate of €900 and a special repayment of €2,000. Then the interest rate can rise above 8% and you are still finished faster than 25 years (without special repayments after 15 years). If you can still pay the €2,000, then you’re done after about 22 years despite 8% interest. With €3,000 annual special repayment, you’re already done after 20 years. The building society saver, even with €2,000 special repayment, does not have time to exploit its advantages for you because the term is too short. Also, without a building society saver, the risk is gone that the building society saver might not be able to cover the residual loan after 15 years because no special repayments were made. The building society saver probably won’t have much more than €40,000 after 15 years. In my opinion, building society savers are all just sugarcoating of interest rates. That they are paid on a much higher amount, the bank does not reveal. Are you assuming that the interest rate will rise even higher than 8% and you won’t make more than €2,000 annual special repayments? If yes, then the variant may fit; if not, then leave the building society saver out.