Hi @clement,
The Cesium writer will automatically reproject the output data to the EPSG:4978 coordinate system, as long as the data sent to it has a valid coordinate system. Please try removing the CsmapReprojector transformer.
If you do wish to reproject to a Geocentric coordinate system like EPSG:4978, you must pick a Vertical mode as well, since the elevation is reprojected as well. The best setting for most source coordinate systems would be 'Heights are relative to the ellipsoid(s) or geocentric'.