Why is this the most profitable option?
Because there is nothing more profitable than reducing your debt burden as quickly as possible. The savings from this grow exponentially.
This is about the regular repayment included in the monthly installment. Doesn’t that contradict the first sentence?!
The repayment includes both: special repayment + regular repayment.
However, the ideal would be to keep the possible special repayment unlimitedly high and the fixed agreed repayment as low as possible. Then you could repay your loan as quickly and flexibly as possible depending on your capability, and at the same time, in case of financial problems, you would mostly only have to bear the interest burden.
The opposite and disadvantageous model for you is the fixed rate without the possibility of special repayments or changing the repayment rate. There, you have to keep a safety buffer (extend the payment period longer than necessary) and depending on the agreed repayment rate, you could more easily get into payment difficulties.