KfW as a bullet loan with a 4-year term

Dear forum community,

The following is rather theoretical and I would like to know the opinion of experienced people whether such a financial structure is possible and practicable, i.e., whether the involved institutions would participate. Maybe someone has actually done something like this before.

Situation:

House construction: Single-family house approx. 150m², Energy Saving Ordinance standard, 4-5 persons, no basement, heat pump + underfloor heating (possibly + BKA), photovoltaic, location NRW
Costs: ~400k€ without land and without photovoltaic
Equity: 200k€

Currently, for a 200k€ loan, we get an effective annual interest rate of 0.75% fixed for 15 years thanks to equity, including repayment rate changes and up to 5% prepayment per year.

The KfW also offers 0.75%, but only for 120k€ and max. 10 years fixed, and it is generally a rather rigid structure. No repayment rate changes, no prepayment possible. Because of this, despite the repayment grant, it was rather unattractive to us, especially considering that an energy consultant is also needed (who is partially subsidized, but not 100%) and possibly the heating energy demand must be reduced through various measures (insulation, controlled residential ventilation + heat recovery...); so far, the additional costs have not been offset by the grant.
Now the grants have increased significantly. I have not calculated it exactly yet, as our planning is not far enough, but perhaps an improvement beyond the Energy Saving Ordinance to KfW 55/40/40+ standard could now be cost-neutral. Possibly, KfW40+ is even interesting, since photovoltaic is planned anyway, so it would "only" require more insulation + controlled residential ventilation + storage, but up to 30k€ grant would be available for this.

And now the theory:
House costs: 430k€ (assuming the upgrade is cost-neutral), plus photovoltaic as above.
KfW40+ loan of 120k€ as a bullet loan with 1 year repayment-free period and total duration of 4 years. Repayment grant 30k€. Interest costs 3 * 90k€ * 0.75% = 2025€.
Equity: total 200k€, used as follows:
- 110k€ immediately
- 90k€ to repay the KfW loan after 4 years.
What remains is what I still have to borrow from the financing bank: 430k€ (house) - 120k€ (KfW) - 110k€ (equity) = 200k€, exactly as before. The interest conditions should not change much, provided the financing bank treats the KfW loan as equity.
Difference:
Significantly more energy-efficient house, thus lower operating costs and better protection against rising energy prices.
Controlled residential ventilation + heat recovery, which is mainly required due to the high insulation but also contributes to increased living comfort.
Not to be omitted are still the not 100% subsidized energy consultant and the interest costs for the KfW loan of about ~2k, and also blower-door test, etc. But other subsidies are also not taken into account, such as 1000€ for controlled residential ventilation via progres.NRW.

Questions:
1) Do I have a major conceptual error, or would all this be possible as described?
2) Is an upgrade from Energy Saving Ordinance to KfW 40+ for 30k€ plus photovoltaic realistic?
3) How likely is it that the bank treats the KfW loan as equity?
4) I still see optimization potential in the fact that 90k€ equity would lie dormant in my account for 4 years. Instead of a bullet loan from KfW, one could run an annuity loan over 4 years with the largest possible repayment. This would reduce interest costs somewhat. Are there more sensible alternatives (I do not consider spending the money on further upgrades or other things as sensible in this context)?

Thank you for your time and your answers.

Is the plot already available?

How do you want to build?
For some options, KFW40 is simply possible; for others, it is very expensive and complex.

I don't quite understand why you want to bring in the equity only after 4 years; the KFW loan is not more expensive than the main loan?!

The property is a leasehold property. It will be a solid house, approximately 9x12m in footprint. As mentioned, the target is basically only the Energy Saving Ordinance 2016 standard.
Of course, I can also put in the 200k directly, then I borrow 120k from the KfW and 110k from the bank. After 10 years, the KfW loan will get new interest conditions. Where they will be is unknown now. If I want to hurry with the KfW loan because of that, there will be hardly anything left or nothing left for repaying the 110k with the bank. Nevertheless, the KfW loan will probably not be repaid after 10 years, so there will still be a residual debt. In parallel, I pay interest for 10 years on the 110k from the bank with no or little repayment. Therefore, I would probably have to choose a 20-year fixed interest period there, but that makes the interest more expensive again (which I pay for 10 years without significantly repaying).
By the way, apparently the conditions do not improve again if I put in 320k€ of equity effectively from the 430k€ purchase price with a KfW loan. Then the interest rates rise again. Apparently, with my loan-to-value ratio, I have already hit the bottom.

The KFW loan is NOT counted as equity by the bank. Why should it be? It is a loan, not capital.

You can optionally take out a forward loan that will replace the [kfw Kredit] after 4 years. I don't know how much extra the bank charges for that. But if I were you, I would just take the 10-year [kfw] and be done with it.

The KFW loan is NOT counted as equity by the bank.

This is exactly what I have read in various places. Probably not every bank does this by far. But since the KfW loan is usually registered as a second charge in the land register – as is also the case with building society loans, which are normally accepted as equity without any problems – I at least do not consider this to be impossible.