Securing interest rate for KfW 124 financing

You can also make a pre-contract on a fund basis

I only took out a "Mini Zins Bausparer" with LBS last week. During the loan phase, it has 0.99%...

..isn't the interest rate after 10 years with a remaining amount of 30,000 just completely irrelevant?

That is correct. The difference between an interest rate of 1% or 5% on a remaining debt of €30,000 is (initially) €1,200 per year. Why anyone would want to secure this in any form is a mystery to me. Especially since the KFW tranche is the cheapest financing component. You put free money into the most expensive part to save as much interest as possible, instead of securing the smallest, cheapest tranche. Or to put it another way: Would KFW 124 still have been cheaper, considering the now planned hedging costs, than, for example, a normal tranche from the bank tied over 15 years?

That is correct. The difference between a 1% or 5% interest rate on a remaining debt of €30,000 is (initially) €1,200 per year. Why anyone would want to secure that in any way is a mystery to me. Especially since the KFW tranche is the cheapest part of the financing. Free money is invested in the most expensive part to save as much interest as possible, instead of securing the smallest, cheapest tranche. Or to put it another way: considering the now planned hedging costs, would KFW 124 still have been cheaper than, for example, a normal tranche from the bank fixed, say, over 15 years?

Can the repayment be increased retroactively?

Free money is put into the most expensive part to save as much interest as possible, instead of securing the smallest, cheapest tranche.

That's exactly right! If you start a building savings contract now, or put the money aside, it practically brings no interest. At least if the investment is supposed to be risk-free – and it has to be, because in 10 years I will need the corresponding sum. I myself have just taken out a building savings contract, the credit interest rate is 0.1%. It makes more sense to put "extra" money into the main loan (special repayment) than to put 30k€ aside and otherwise obediently pay interest.

I have also asked myself the question of how I will refinance the 33k€ left over from my KfW loan (I have the same loan). That is actually too small an amount, at least my bank advisor told me that she usually only becomes active from 50k€ onwards.

can you increase the repayment afterwards?

It depends on the contract.

With KFW 124 everything is fixed in advance.
Partial repayments are possible against a prepayment penalty.