Removing static at the wall

I can only strongly advise you to go to a structural engineer. My brother also had this properly calculated for his similarly aged prefabricated house. In his case, about 6 meters of wall were removed as well. But there, a really hefty beam with lateral supports was installed.

And it's not always just about load-bearing, but also about bracing.

Overall, no one here will give you advice, and without a structural engineer, you are playing with your life and your assets.

Can such walls even be load-bearing? With this construction, I don't even trust the exterior wall to properly carry any load.

Yes, otherwise your roof would be floating. The wooden studs carry the load.

Every single wall on every floor is taken into account in the structural calculations. That would mean every wall is structurally relevant. That can't be right.

Yes, it is.

I have heard that 6 linear meters can be spanned without support (pillar, wall...) - is that true?

In certain constellations, yes. But these are then accounted for in the structural calculations beforehand, and the other walls carry the loads. Special beams are installed for this, etc. But that doesn't mean you can just remove any wall up to 6m wide.

Actually good chances for a safe removal of the wall, or am I missing something?

Yes, the structural analysis.

Again: go to the structural engineer.

Here in a neighboring town, a house once collapsed on a young couple because they removed walls without a structural engineer. The building insurance subsequently refused to cover the damage. The outcry and calls for donations were huge. I can only call that stupidity.