Bezüglich des Parkens am Straßenrand auf einer Landstraße, wenn in der Mitte keine Schmutzlinie vorhanden ist

What exactly is your question?

You don’t need a line for parking. If it has a good visibility in both directions and it is not on the hill, you can assume the central line would be striped (allowed to park to the side normally). If you think a solid line would belong there (bad visibility, on a hill), then you must park the car 3 meters from the assumed line/center of the road.

https://www.autoliitto.fi/uutiset/pysakointisaantoja-on-noudatettava-myos-taajaman-ulkopuolella/

You cannot park there without hindering traffic as they would need to go to opposite lane to pass your parked car. So no. But white line in the center isn’t the deciding factor.