Is the overall cost estimation realistic?

Here is just my personal opinion:

I would fix the interest rate for at least 20 years or even consider a 25-year interest rate lock. Sure, you pay more interest, BUT and now the important part: You don't know what will happen or could happen in the future. And then jumping into the risk of an expensive follow-up financing, although you get cheap money now. Eight years ago, everyone was talking about cheap 4.5%... Parallel saving for repayment only makes sense if you include it in the financing every year (special repayment).
But we also do it like you, keep the repayment as low as possible, but have a fixed savings amount and at the end of the year put everything in as a special repayment. We want to be flexible and still be able to afford things or sometimes use a bonus for special repayment.
Best regards

The 2% of the follow-up financing are, however, something completely different (namely significantly less) than the 2% of the original loan, because the initial amounts differ significantly and, moreover, the repayment portion of your annuity will have increased, so you will then repay more than 2% annually. So you can't calculate it like that (or it doesn't lead to anything meaningful). What exactly is the goal of your calculation? What do you hope to compare?

I don't quite understand, what would the rate be in my example then? I just wanted to estimate the risk if the interest rate increases significantly

If you have an annuity loan, your repayment increases from an initial 2% over time. If you pay the same amount every month, the interest burden per month decreases as the loan is repaid - and since you are paying constant amounts per month, the repayment correspondingly increases.

If you enter the follow-up financing and reduce the repayment to 2% again, you could consequently have the same rate as before despite significantly higher interest rates.

There are annuity calculators on the internet that calculate this down to the last cent for any basic assumptions.

I don't quite understand, what would the rate be in my example then? I just wanted to estimate the risk if the interest increases significantly

Under what condition? Same total term? Same total cost? Same rate (if possible)? Same repaid amount as at the end of the first fixed interest period? The answer varies greatly depending on that. Banks usually calculate afaik with the same total term, but that is not mandatory (and depending on the case, not necessarily sensible)

Under which assumption? Same total term? Same total costs? Same installment (if possible)? Same amount repaid as at the end of the first fixed interest period? The answer varies greatly depending on that. Banks usually calculate afaik with the same total term, but that is not mandatory (and depending on the situation, not necessarily sensible)

I want to be able to estimate how the installment will develop with certain interest rate increases. To find out if we can afford it even with a possibly much higher interest rate. What’s important to us is that we are not paying off forever, but finished by pension/retirement. I will try this out with a few annuity calculators... having around 200,000 EUR remaining debt after 15 years (I will then be 41 years old) sounds like a lot

In my opinion, the calculation would be as follows:
€200,000 outstanding debt with a follow-up financing of, for example, 6% and a monthly installment of say €1400:
At the beginning:
Interest portion €1000, principal repayment €400

in the next year:
Interest portion €998, principal repayment €402

from the 11th year:
Interest portion €672, principal repayment €723

The loan would therefore be repaid after approximately 21 years, not taking into account special repayments.
This would give you a total duration of 36 years; given your age, this fits until retirement.