Building as a single, marriage, and prenuptial agreement

So if one partner is very frugal and the other throws money around, it can actually lead to resentment when you think about it carefully.

Exaggeratedly said, one saves and invests cleverly and the other gambles the money away in the casino. With a joint account, you can't really separate that at all, but with 2 separate accounts you can, yet it still doesn't happen. Theoretically, the saver is even punished because they hand over half of the savings even though in recent years they may not have bought the new iPhone every year or the most expensive clothes, car, etc.
This is not specifically about me, although I am very focused on saving and investing.

I don't understand the last part of your post at all,

From where exactly?

So we just find a specialist lawyer and discuss on site how to arrange what, sign it and then the story is done?

Even a good specialist lawyer can be a lazy dog, and in any case, his experience cannot replace the fact that you already go there with your own active idea of the content. Otherwise, in the end you will get a block text collage as a combination of 08/15, Template F and as little work as possible. A contract is there to secure reconciliation even for the event of a dispute. The fundamental hypothesis here is therefore to start from broken rose-colored glasses, or more concretely aimed at you: assuming she would have bad traits in dealing with money and maintain all of them even with a joint account, how could the marriage then remain intact?

A prenuptial agreement is worth nothing in the worst case before the wrong judge. They wipe their ass with it if anyone is disadvantaged. Before marriage, one should rather rely on self-determined factors like a long-term relationship (5-10 years), possibly having survived the stressful "first child" phase, possibly building a house, and then there is still enough risk, but by then you have survived a few ups and downs and can finally invite Father State into the relationship.

Your question at the end of your post is interesting. I have never really thought about it since we have separate accounts and everyone does with their money what they like without somehow making debts for consumption. But what could a regulation in a marriage contract look like in this regard?

The fact is that neither of us will bring any significant cash amounts into the marriage, I’ll leave aside the small emergency fund.

I earn roughly twice as much, and as a result saving is obviously much easier for me than for my wife. No idea if that plays any role.

As already mentioned in a post - take a look at the normal, legal regulations. If they fit, then everything is fine. Only the house rule might make sense if applicable. Marriage is simply, quite unromantic, a joint economic form with mutual rights and obligations. You then form an economic unit together. As everywhere - a separation is unpleasant and often a problem. Together you are simply stronger, even if in the internal relationship of income it is 2:1.

Assuming she had bad traits when dealing with money and kept them all even with a joint account, how could the marriage remain intact?

Quite simple – by not taking money too seriously. Lack of money for living is a curse. If there is enough to live on, please don’t take it so seriously anymore. Money comes and goes. Time does not. Just focus more on good moments and a good time together, for each other and for yourself – and then different spending behaviors really aren’t dramatic anymore.