You can use pconvert directly from the command line. It is shipped along with SNAP. The usage is documented in the help.
Then you can avoid python at all or you can call it from a Python script as a sub-process…
Or you use snappy ( Configure Python to use the SNAP-Python (snappy) interface - SNAP Wiki). Then you can have a look at these threads which can help you:
Exporting RGB image as GeoTIFF error - development / python - STEP Forum (esa.int)
Exporting RGB image with vector layers - development / python - STEP Forum (esa.int)