I have a suggestion for you that boils down to implementing your project as originally planned, even if that doesn’t necessarily align with the majority opinion here in the forum. You yourself wrote that you are a security-oriented person. Comprehensive insurance is always expensive. But maybe I can give you some other input.
You already have a paid building plot. Bravo. And even from the municipality, so hopefully it wasn’t overpriced. In my circle of acquaintances, many are looking for a municipal plot and often fail due to local social point systems. You’ve already taken the first step.
Now a nice family home is to be built on the plot. Various sizes have been discussed here and were tendentially deemed unaffordable given your income. I would not build with an architect; from my point of view, that’s a waste of money. You will definitely find a general contractor who will build you a solid masonry house that fits within your financial framework. It won’t be quite as individual as with an architect, but you can still realize yourself to some extent. By the way, 140 to 160 sqm should be enough for a family of three to four. You should calculate a 10% buffer (or higher depending on your personal sense of security) into the fixed price.
Significant potential to avoid financial collapse in the coming years given your specific family situation: you don’t have to amortize 2 to 3% right from the start. Take it slow. Yes, there is also the 1% amortization with a bank with an orange logo. Plus the KfW financing—when I applied, there was even a several-year amortization-free period possible. Of course, after 10 years you will still have a high remaining debt. But you are young. Salary increases should still be possible. I also read something about a significant inheritance later on, which could then allow special repayments. Finance the thing with 1% amortization over 15 years, then you can later see if higher amortization rates and/or special repayments are possible. If things get really tough (interest rate increase), you have security for 15 years at first and could then continue financing at higher costs at least until the children are out of the house. If you don’t build in a demographic “loser region,” you can sell the house for a profit if necessary. Many want to completely pay off their property by a certain day and forget to live in the meantime. Who knows what will be in 20 / 30 years? Where you want to live/work, whether you’ll need the space after the children move out. Also a point: whether the partnership lasts that long. I see it this way: you can, but you don’t have to plan everything all the way to the end of life. From my personal experience, that always goes wrong anyway.
The advantage if you follow through: you will have lived beautiful years with your children in a great environment and can later reorient yourself or not. In most regions, even with sale after 20 years and only little amortization, there should still be a profit due to appreciation.
Best regards