Would you briefly explain why the "large" home savings contract is supposed to be bad? Briefly on this: 100TE savings amount, 1% interest, 2.2% fixed interest rate, can be used from 40TE savings, 0.5% commission (Signal Iduna)
It simply does not fit your needs. What you need to do now is save more equity. However, there are other safe investment forms that work without fees and have no cancellation periods.
Assuming you would soon find a cheaper property or have simultaneously saved equity to actually enter into a real estate financing. How would your home savings contract help you then, which does not fit the property amount-wise and whose duration is also difficult to plan? In financing, you would need to provide a component that at time X can have exactly €100,000 remaining debt and be due so that it can then be paid off with the home savings contract. Unfortunately, the allocation time of the home savings contract cannot be precisely guaranteed. Timing these together is accordingly difficult. Parallel to repaying your loans, you would then have to continue paying into the home savings contract instead of reducing your debt load. That costs money in the form of interest.
Furthermore, your home savings contract, which is to be saved with €40,000 at a monthly saving rate of €300, will be ready for allocation in about 11 years at the earliest. Integrating that into a financing that should preferably start now (unfortunately likely not) or soon (1-2 years) is very questionable both regarding the amount (€100K) and the duration (completely uncertain). You need the money now, not in 11 years. It is not the current income failing, but the cash availability.