You're probably far ahead of me on this topic, but as an immigrant to Germany for some years now, living with a German partner, I'm convinced that Germany (at least, Bavaria) has its own specific cultural norms that makes socialising far more challenging than is my experience in other countries. I can't explain why, but I observe a more insular (i.e. family- and existing-close-circle-focused) attitude, and also a significant level of difficulty/inflexibility regarding scheduling social events.
For example, I often find it quicker and easier to agree the timing and details of weekend trips to meet up with friends in other countries, involving one or both sides traveling significantly to meet, than arranging a single evening to meet for dinner with a single existing German colleague or friend living nearby. Of course these people have lives and arrangements I must fit in with, but I'm convinced that the examples I'm thinking of do not have such overwhelmingly busy schedules as to explain the observation.
This might sound like a trivial observation, but I suspect that the overall effect, if you scale even a small fraction of this behaviour across a whole country, could be huge.
As 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.
For example, I often find it quicker and easier to agree the timing and details of weekend trips to meet up with friends in other countries, involving one or both sides traveling significantly to meet, than arranging a single evening to meet for dinner with a single existing German colleague or friend living nearby. Of course these people have lives and arrangements I must fit in with, but I'm convinced that the examples I'm thinking of do not have such overwhelmingly busy schedules as to explain the observation.
This might sound like a trivial observation, but I suspect that the overall effect, if you scale even a small fraction of this behaviour across a whole country, could be huge.