Evaluates my financing plan: new building + rented DHH with intra-family sale/buyback

We (couple, Bavaria) are planning a new building (KfW-40, ~1.2 million construction budget, property 2026 from EK, construction start in 2027) with a granny flat. There is also a rented semi-detached house (DHH). I would like to have you break down the plan before we go into the bank discussions. **Initial financing ~825k over 4 loans:** Loan amount conditions DHH (rented) 250k 3.7% + 1% Bank 1 (new building) 375k 3.5%, 10 years, 1% repayment KfW 1 + 2 each 100k 0.6%, final maturity - EK ~750k, loan-to-value ratio new building ~40%, Ra