Fixed interest rate for the entire term or only 10 years?

Yes, €500 are saved so that it is ready for allocation after 28 years.

So you save for 28 years and three months (in the previous post it says 18 years), earn no interest on what you have saved but always pay full interest on the entire loan amount (15 years at 0.6% and afterwards 2%), and then use the savings to repay the loan? Crazy suggestion and such a thing CAN NEVER be cheaper for you in life than the full amortization loan itself even at 1.8% interest.
At least do option 1 and instead of this deferred repayment loan, take a 30-year full amortization loan!!

If I calculate an annuity loan with €250,000 and 1.8% over 30 years, I come up with interest of €73,700. So it is more expensive than the €44,400 from the building society savings contract

And the regular full amortization loan is cheaper. Don’t get fixated on the building savings contract – they are almost always rubbish.

What do you mean by normal full amortization loans?

A loan with the appropriate repayment so that you are done in 30 years. No repayment substitute, but a classic annuity loan without any gimmicks.