Real estate repayment investment?

I have an offer for a KFW real estate loan, where after the interest rate guarantee expires in 10 years, I will still have a remaining debt that I plan to pay off completely at that time.
I am saving the amount for this with my annual holiday bonus, since this is a one-time payment, I would prefer not to increase the monthly repayment rate.
Special repayments are not possible with KfW for this matter.
Now the question is, which fund is best suited for such things? Of course, I could invest in an ETF, but there I obviously have bad odds if the price is poor in 10 years and this cannot be offset by the returns of previous periods.
The only alternative that comes to mind right now is daily or fixed-term deposits, where I at least have a guaranteed 1% long-term currently.
What other option can you think of?

Building savings contract

Hey , or help help... were you banned or why the second nickname?

Just no building savings contract. High costs, poor interest rates. Better under the mattress.

ETFs and sell in time (1-2 years in advance) when the price is good. A savings plan evens out losses on average. Set yourself a return target, avoid greed.

How much do you want to save? 1% fixed deposit interest is quite low, hardly worth the effort, but better than nothing.

Sell ETFs in time (1-2 years in advance) when the price is good. A savings plan averages out losses. Set yourself a return target, avoid greed.

How much do you want to save? 1% fixed deposit interest is quite low, hardly worth the effort, but better than nothing.

The savings amount is about 3000-4000€ per year. After reconsidering, I thought possibly about a mixed calculation of conservative investments like fixed deposits together with a lead time plan and a small portion of riskier investments like ETFs, whose losses would then partially offset the planned returns of the other things.