How to convert CityGML (LoD2) to Digital Surface Model?

I’m very new to GIS and doing this for a university project. For evaluating Solar Potentials on Building Roofs I want to use QGIS 3 (the UMEP Plugin, with SEBE tool). Amongst other data I need a Digital Surface Model preferably as a .tif file. For that I already have elevation data but where I’m really struggling are the CityGML files. The goal is to get moderately detailed information about the solar potential on differently shaped roofs in their geographic environment, hence roof shapes, position towards the sun and shades are very important.

Here is my problem:
I want to merge the CityGML (LoD2) buildings on the elevation map and from that create a 2D Digital Surface Model, with accurate representation of heights of the roofs. The DSM should reflect the shaping of the roofs not only cubics or similar.

I though about gdal_rasterize but am convinced it won’t help me too much as it doesn’t work for CityGML as far as I’m concerned. I also experimented a bit with CityJSON but at the moment I’m running out of ideas on what to google.

The building data is from this page (unfortunatelly in German):
https://www.hannover.de/Leben-in-der-Region-Hannover/Verwaltungen-Kommunen/Die-Verwaltung-der-Landeshauptstadt-Hannover/Dezernate-und-Fachbereiche-der-LHH/Baudezernat/Fachbereich-Planen-und-Stadtentwicklung/Geoinformation/Open-GeoData/3D-Stadtmodell-und-Gel%C3%A4ndemodell/Digitales-3D-Stadtmodell