Hüttig und Rompf is specialized in the construction areas and did that for us. Our house bank couldn't keep up.
Well, generally it's not ideal to put all your eggs in one basket (intermediary). As far as I know, the Schufa score has nothing to do with the interest rates. I have now read everything again. Your creditworthiness is probably the problem. For the loan amount, you simply earn too little in my opinion. You can also tell by the fact that you have only agreed to a 2% repayment. I assume that for a higher repayment, the bank's household budget simply wouldn't have fit anymore. Against this background, I would say the conditions suit you. You are basically getting about the worst terms currently available on the market. Basically, you can almost be glad that anyone is financing this project for you at all. :)