Convert 3D-Buildings as MultiPolygonZ to 2D shapefile with height feature

As of March 2021 Switzerland provides the swiss 3D building geodataset for free, it is available as dxf or gdb, the latter contains MultiPolygonZ geometries (buildings also with complex roof geometries). I want to convert these to a 2D shapefile, where building height is a field. I see that the derived geometry in the attribute table contains an interpolated Z and a Next support point Z (in german: Nächster Stützpunkt Z).
Is there a tool or process in QGIS to reduce the 3D dataset to 2D? Or would it be easier to iterate over every MultiPolygonZ in geopandas, keep the extents and average the uppermost z support point values?