Property debt-free - Loan on man?

The house becomes "too big" for a "normal" family (not trying to sound arrogant) – should this have any impact regarding parental support.

Certainly, because it results in a "housing advantage," regardless of whether married or not, right? But is the income even high enough? (see last paragraph)

What also interests me a lot is whether there is a way to represent the matter cleanly without a land register entry. A question that often comes up: the loan contract only in the man’s name? Although then I am uncertain how this should be represented properly – in case of a separation, only one person is left with the debt.

Without the woman being listed in the land register, I would never recommend that the woman sign the loan contract. Because even after a separation, the woman remains liable with income and assets for a house that belongs to her 0%. Whoever signs something like that is beyond help.

If you marry, it is usually forever

Well, currently 40% decide that "forever" is too long. But it always only happens to the others ;) But now seriously: Don’t you only have to pay starting from €100,000 gross income of the child, regardless of the spouse? Is that much actually earned when no equity is available at all?

we want to get married in the next few years, but with a prenuptial agreement because of her obligations towards her parents. (nursing home, regularly receives mail for income disclosure - we would "earn too much" together)

Congratulations to your wife. If she has to pay, it means she earns more than 100k gross per year.
There was a reform in 2020. How much you earn together is completely irrelevant. Only your wife's income counts.
Any average lawyer should be able to dissect the letter in 10 minutes if her gross salary is below that.

Since the property already belongs to one person, you should establish an ownership ratio. So let's say you pay half for the house, and the land accounts for half of the price of the land, then that would be 1/3 to 2/3.

Then one "sells" the other 1/3 of the land by registering this 1/3 in the land register. For the tax office, this is considered a gift, which has a very low allowance for unmarried couples. However, it can also be done after marriage, then the allowance is considerable.

we want to get married in the next few years, but with a marriage contract because of obligations she has towards her parents. (Nursing home, regularly receives mail regarding income disclosure - together we would "earn too much")

You do know that this has changed since the beginning of the year? Since then, the so-called [Family Member Relief Act] applies: children with an [annual gross income of less than 100,000 euros] are no longer obligated to provide maintenance to their parents. The decisive factor is the annual income, not the child's total assets. That means only children with an annual income of more than 100,000 euros are generally obligated to support their parents in need of care. And here, AFAIR, the rule applies that children are only considered with their own income, but not that for married couples the income is simply divided by two, so the higher earner would indirectly have to cover the care of the parents-in-law!

how does an unmarried couple divide the matter most fairly?

He sells his half of the land share to her. She must (somehow) raise her share. Then joint ownership and joint construction financing. Alternatively, you don’t divide at all and he remains 100% owner. Then he must also take on 100% of the financing.

we want to get married in the next few years, but with a marriage contract due to her obligations towards her parents. (Nursing home, regularly receives mail regarding income statement - together we would earn "too much")

Is this still the old or the current status regarding parental support? Parental support was reformed in 2020. The income limit is now at 100K€ for the child. Only when the 100K€ gross annual income is exceeded is the ability to pay calculated, and the partner’s income must also be taken into account there. (Note: layperson’s knowledge) PS: Scout was faster