Transfer property according to share

Then make a private (interest-free) loan agreement for the money she contributed.

In case of separation, you have to pay her back the money. How you do that is your business. End of story.

Repeat the same once a year for the ongoing contributions.

In my opinion, this is the leanest and best way.

Because with the statutory matrimonial property regime, everything is split half and half in the end.

This only applies to the accrued gains.

What do you mean by disadvantaged? In the event of a possible separation, the house is on my property and her equity (existing before the marriage) is invested in it. I alone decide what happens to this property. This is exactly where I want to give her rights. I'm not sure if the legal requirements are sufficient here.

Somehow you don't understand this correctly. If the house costs sum X, everyone gets half of X after the divorce. The previously invested equity is then offset against the value of the property and possibly balanced out. All completely independent of the land register. The land register only says that the house belongs to you, but not that you don't have to pay her 50% of it, wherever you get the money from.

Exactly... You need to organize and sort the source of the funds, not the container they are in.

Currently, before marriage, it is the case that as the landowner I also own everything that is built on it.

If you are (still) not married, in my opinion it is not that simple. One should take the time to sort it out carefully and consider what one wants to achieve. Then one can think about how to approach it.