Export View as Image (Snap)

i am trying to export an Image by using the “Export View as Image, Full-Scene” (e.g the result of the ndvi-poecessor) as geotiff, which works very vell. Unfortunately the data type of the geo-tiff is always 8-bit, is it possible to change the data type to float16 or float 32?

No not in this case. This is an image export.
You can subset your result (spatially and band subsetting) to contain only the data you want. Then export
by File / Export / GeoTiff. Then you get the NDVI values.