Car parking space on gravel turf, grass grids, etc.

How about just sowing grass first? And then analyzing based on the usage and tire tracks what kind and how much firmer surface you actually need? Sometimes it is the case that you only need a strip of firmer surface. Recently I saw at a house that they "simply" just laid very nice (stepping?) slabs into the lawn where they parked one of the cars. Seemed to be enough and looked very good in my opinion. With a correspondingly robust lawn, this might be tried out first.

The problem is that topsoil on the one hand does not support well when wet and on the other hand gets compacted by driving on it. As a result, water stands and it becomes muddy. It doesn't even have to be as much traffic as at Wacken. If you just lay slabs on the topsoil, that will at best work with sandy subsoil. That is why there is gravel lawn. There the gravel supports and drains, but because of the soil content, grass also grows. You don't even recognize it as such.