Agree with most of your comments, and also have some experience in Germany with Germans. No claim to expertise on the subject.
Let me suggest that European culture of 1914 resulted in large part from the complete defeat of Napoleon in 1815.
In 1815 the Old Order was then dead, a new order rose, prospered and dominated the world.
In1865 the American Old Order was completely defeated, a new order rose, prospered and dominated North America.
In 1919 the old order was only partially defeated, and in its place began to grow several different possible competing new orders.
From international socialism to national socialism to democratic socialism to...
They were all more or less socialists, which means at war with their own people, robbing themselves of cultural vigor, PLUS soon at war with each other, beginning in Spain.
Let me put it this way: “cultural decline” is a self inflicted wound — an autoimmune disease, where the body politic attacks itself in a basically insane effort to be rid of imaginary pathogens.
I think precisely that, rather than some post-romantic “disillusion” or angst explains Europe’s declines, cultural, political, etc.
We can debate this endlessly, probably will, and will thoroughly enjoy it.
I see history as “continental drift.” The underlying forces push nations and peoples in various directions. There is resistance. The pressures build up against the resistance, and then you have an “earthquake,” or in the human context, war. After the war, the pressures have been spent/released, a new order settles in, and then the pressures start to build again until the next earthquake. And, in history, too, there are “fault lines.” It’s a nice analogy, but I will concede that it only goes so far.
As far as underlying pressures are concerned, there can be many of them. Migration, in part caused by climate cycles, has been one of them. Resource development, uneven technological progress, population growth/disease....the list could be endless.
In the context of 1918, the blight of socialism was already there in most societies. Some places virulent communism, some places a more quiescent socialism. But it was already there, exerting its pressures. When the “earthquake” of WW1 destroyed or discredited the existing social/political order, socialism was there to gain from it. It was kept at bay in some places better than others. However, I don’t think it would have been successful without the war and the opportunity to spread its lies (”land bread peace” comes to mind) to an exhausted population.
The two competing strains of socialism fought the next war, with the nationalists losing to the internationalists. And while we thought we were still capitalist, the forces of internationalist socialism were rotting us out from within.