How to proceed to find the suitable financing partner?

Yes, I agree. Thank you.

Now another question. Of course, we want to determine the "most favorable" conditions for us (in terms of personal circumstances). However, I have also read that one should be careful with a loan inquiry or condition inquiry, since the former is recorded in the Schufa and visible to everyone. So if I were to approach 5 banks with our plan, would that be a condition inquiry or a loan inquiry?

Request for terms, no Schufa.

And even if there is one, as long as there is no negative entry, no one cares about Schufa.

We have 3 construction financing offers and now I need an "INDEPENDENT" person who can take a look at everything and give me their honest opinion. Maybe someone knows one. Please PM

Just post them here and you will get feedback. If you are looking for something "professional" and "independent," then you will have to pay for it, because the intermediaries earn their living from brokering, not from checking third-party offers. Or get an offer from an independent intermediary as a comparison.

A bit late but:
The consumer advice centers offer something like that. We once had a consultation for home financing – not a contract review, but an assessment of personal financial possibilities. The bank also does something like that in the discussions, but they always want to sell you something, so they may not be as neutral. The consumer advice center only earns from the consultation (€170).
I read in a book that it is better not to fill out the banks’ standard forms for the rate inquiry, but to write something yourself, similar to a cover letter, listing all the necessary information (salary, living expenses, insurances, equity, information about the property, etc.).
Has anyone done something like that here? Is that actually advisable or can I save myself the trouble?

I think you can save yourself the effort. Banks are interested in numbers, data, and facts, not the individual person behind them. The employee in the credit department will probably roll their eyes if they cannot quickly extract the necessary information from their familiar form, but instead have to laboriously pull it out of an individual letter in long text. At least that's how it would be for me.