Evaluate financing model? Possibly cheaper?

Hello everyone,
I am Robert and as a single person I now want to start building my own house. For this, I have already chosen my plot of land and it will be purchased next week. To find out what I can afford and how to approach the financing, I have already obtained a few offers in advance and somehow I receive different advice and financing models everywhere.

To get to the construction project first, my plan is as follows:
Equity:
50,000 € - Plot including incidental costs
25,000 € - Riester savings plan. Conversion into Wohnriester.
10,000 € - Personal contribution

After some research and recalculating the financing amount, I have come to a maximum affordable monthly burden of about 1,200 € for my circumstances and the fact that I have to manage everything alone (Current net monthly income: 3,100 €). Based on the financing comparisons I have received so far, this results in a maximum purchase price including plot, incidental costs, and outdoor facilities of 400,000 €, of which I have to finance 315,000 €.

Among the offers I have received so far, there were full financing over 30 years, as well as financing with interest rate locks of 20, 15, and 10 years with interest rate protection through a home savings contract. KFW loans were mostly already included.

My personal assessment is that the interest rate games with home savings contracts make the entire financing confusing and complicated. Even though the interest rates initially appear very tempting, I really struggle to see a real advantage here despite having very good math skills. I am currently leaning towards financing with a 20-year fixed interest rate. All offers I have obtained for this model so far have an interest rate between 2.2 and 2.4%, as well as the KFW loan of 50,000 with a 10-year fixed interest rate (1.15% - 1.55%).

What do you think of my model?
Do you have suggestions on how to possibly realize the financing more cheaply?

Now to my actual question:
I am currently still thinking about how to secure the respective residual debt interest-wise after 10 or 20 years. I consider a home savings contract at least the most interesting option for the KFW loan. For the 20-year financing, I am wondering whether I should extend to 25 years or take the interest rate risk? Do you perhaps see better options in my case? How do you currently view the interest rates offered by you?

Thank you very much

For your preferred model; what do you pay per month and how much remaining debt would you have after 20 years or after 25 years?

You haven't said anything about the initial repayment.

Yes sorry that is missing so the initial repayment is 2% each time
thus the remaining debt based on the example: 2.38% / 1.55%:
20Y-loan: €129,412.59
10Y-KFW-loan: €33,390.04

with 25Y at an interest rate of 2.55% there would still be about €80,000 remaining debt.

Hello, do you really want to finance a house as a single person at 37 years old with 2% repayment where you still have €80,000 left to pay off after 25 years? You won't make it through as a retiree.

Yes, I do! Therefore, my question is not about whether it makes sense. But I will be done after 25 years. My relevant AV with capital withdrawal would cover that, which I do not necessarily want. In addition, I can still influence the term through special repayments. Basically, I am asking about the best option to guarantee interest rate security. That also does not mean that I will remain single until retirement.

With the info, I wouldn't secure anything further there. For the KFW component, after 10 years you will receive a prolongation offer at the then usual conditions. Whether it costs 2 or 4% at 33,000€ credit is not decisive. If you don't feel like dealing with that, consider cutting the component entirely and financing everything with the longer term. I also wouldn't secure the remaining debt after 20 years if your life insurance can cover it. I would pay off the 20 years, possibly make special repayments. In 20 years, the salary will look different anyway due to raises and inflation, I wouldn't overestimate that today. The money you would put into a building savings contract, you can also use directly for special repayments or invest in a more risk-oriented form with a long-term investment horizon (stocks, index funds, whatever). Loan conditions improve especially through more equity (private loans within the family?) and time investment for comparing offers.