alt Hacker NewsAs a German, I very much share those observations, and it seems to be underdiscussed and not that apparent to most people here.
The German social scheduling culture can definitely be a viscous circle, where everyone having to plan their calendar in advance forces everyone else to plan even further in advance if they want to have a chance to meet up.
I do think it's probably felt the strongest for adults between ~18-35, where your circle of friends spreads out across the country/surrounding cities/world, and any get together necessitates travel. After that, when people settle down (potentially have children) they usually form new circles of friends that are more local again with more opportunities for spontaneous meetings.
So far, the only solution I know is to set a date and stick with it. Trying to find a solution that meets everybody's calendar is rarely worth the effort. First, nobody has time; later, they have other appointments, so in the end it seems to be a matter of priority.
Set your date and stick to it. If people deem your topic relevant, they will arrange to participate.