Is the overall cost estimation realistic?

difficult. The surcharge of 0.46% is not insignificant, but 2.34% is not that bad either. I would calculate everything thoroughly and plan realistic special repayments, then see what remains after the 15 years have expired. Then you can see what the follow-up financing will cost under the worst-case assumption that the interest rate has doubled or tripled by then. In general, it is important to pay down a lot, especially at the beginning. That naturally speaks in favor of option 1. I find the cancellation option after 10 years uninteresting because the interest rate is very unlikely to be lower than the current one in 10 years.

I would choose offer 1 and put the monthly difference of 130 euros compared to option 2 into an account that is really meant for special repayments. Over the 15 years of option 1 alone, that already adds up to 23,400 euros without considering the reduction of interest payments. Ideally, after 15 years, only about 200,000 euros would remain.

That was pretty much my thought as well... I can never really assess the interest rate risk... in 15 years, the interest rate will probably be significantly higher than it is now. We will then have to start the follow-up financing with this interest rate. So can I just pretend in various calculators that we have to borrow 200,000 euros now and play around with the interest rate? Or do follow-up financings have completely different conditions? We are just afraid that the installment will rise sharply in 15 years

You can do it that way, but guessing which interest rate to assume is just speculation. It was already said in 2016 that interest rates would rise significantly again soon... and whether it's 3% or 8% in 15 years...

Keyword interest rate risk: Assuming that after 15 years we still have 215,000 EUR left, the interest rate for the follow-up financing will be 6 %, and we want to continue with 2 % repayment:

New installment:
Interest portion (215,000 * 6 % / 12) = 1,075 EUR
Repayment portion (215,000 * 2 % / 12) = 360 EUR

Makes a monthly installment in the follow-up financing of 1,435 EUR.
You can calculate it like this, right? Interest of course completely guessed. I already find it quite intense if the installment would then just increase by about ~300 EUR.

It is not 200 euros compared to your 20-year offer, and in 15 years there should have been some progress on the income side as well.

and we still want to remain at 2% repayment

The 2% of the follow-up financing is, however, something completely different (namely significantly less) than the 2% of the original loan, since the initial amounts differ significantly and also the repayment portion of your annuity will have increased, so you will then repay more than 2% annually. So you cannot calculate like that (or it leads to nothing meaningful). What exactly is the goal of your calculation? What do you hope to compare?