It's odd that your symbology isn't carrying over. I have a script that converts a series of layers (polygons, polylines, and points) to KMZ and seems to keep it's symbology fine. One thins you could try is saving a copy or your preferred symbology somewhere on your system as a .lyr file and refer to that in your script to set symbology everytime the script runs. Maybe something like this:arcpy.ApplySymbologyFromLayer_management(your_layer, symbology_layer)
arcpy.SaveToLayerFile_management(your_layer, layer_to_be_coverted_to_kmz)
arcpy.LayerToKML_conversion(layer_to_be_coverted_to_kmz, output_kmz,1,"NO_COMPOSITE","",1024,96,"CLAMPED_TO_GROUND")