How can you cool the house a bit without air conditioning?

If the place is suitable for ventilating where it's desired - why not.

do you know a good quiet wall fan?
there wasn't much to find on the internet somehow
some had 57db listed probably at the highest setting and others often 20db probably at the lowest setting

The switch just stays on, the rest is done by the remote control. It's not practical. 2 switches are better.

unfortunately they only have 1 switch and that's very impractical

do you know a good quiet wall fan?

I have not dealt with wall fans yet and cannot give a recommendation.
Regarding noise:
1. The dB value is a volume level that says nothing about the quality of the noise. To illustrate: A quiet squeaking can be more annoying than a louder monotone humming.
2. It is unclear what noise-amplifying properties the wall in your house has.

We have a ceiling fan with light. There are two switches on the fan itself for light and fan. That means if we don't need the fan, you can turn it off on the device, keep the light basically on there, and operate the light via the light switch.

I find you as OP difficult. One thing doesn't work, another can't be done, the next is stupid, the fourth is impractical.

Back to the beginning again. The house was planned, checked, and approved with summer heat protection. If an air conditioner is now installed, it is additionally for personal well-being. No one is going to create a new calculation if an infrared-heated mirror is later hung in the bathroom for well-being. Who is supposed to check a calculation now?

If an air conditioning system is installed now, it is additionally for personal well-being. No one will create a new calculation anyway,

There are two ways of looking at it when you have taken advantage of KfW subsidies. 1. Official: You commit yourself (and also the buyer of your property) for a period of 10 years not to make any changes to the house that would destroy the KfW 55 classification. An air conditioner can be harmful in this regard. Formally, you only know this if you talk to an expert and possibly have a recalculation done. 2. Unofficial: Do it and enjoy it

With a mobile unit, you are on the safe side. Costs little, cools, and has an annoying hose. Considering waiting until the situation is over is of course even cheaper. The heat cannot be so disruptive that you have an acute problem to solve, even if it is very hot right now.