Hi @Jonas,
Sorry to hear you’re having trouble with this.
Thanks @JuGa for your really helpful points here.
Unfortunately, it seems WordPress forces the Index - Home templates to be applied whenever you apply the Posts Page setting for a specific page.
I’ll try and dig a bit deeper in the coming days, but I can’t promise anything as quite a few of the block based functions in Gutenberg are not filterable currently… Something I’m sure they’ll be improving.
Thanks for bringing this up!