Regarding the regulations from 01.07. What does 'eligible costs' mean in the definition of Kfw. This term was always used in connection with renovation under the old conditions. It is also stated as such in the annexes, explanations of Kfw.
Eligible costs are all costs for construction or renovation. I would have to look up in the new BEG leaflets how the declaration will be made from 01.07. and what specific consequences result for individual constellations. In the case of renovation, it is quite simple. For new construction, it can (will) lead to possibly small loans or grants. However, it should normally not matter, since one usually only does "small things" oneself.
Nobody finances undeclared work. It's about the return! Is the effort worth it for just over 5% YES or NO. It can't be that difficult to form an opinion on that.
It’s not worth it. Are you satisfied now? With your setup of almost 100% financing, every bank already says 5% interest.