IMHO,
ISO at 400 is bit wee high.
Try with a lower ISO and maybe an aperture at F9-F16 perhaps.
Experiment with different settings for various effects.
As for starburst effect, you may wish to get a dedicated starburst filter, it should do the trick.
Oh yes, a tripod is a must for long shutter exposure.
Erm. I do suggest reading up a bit and expanding your knowledge before posting something that proves you've been reading Nikon marketing brochures a bit too much. The Sony lenses are very sharp, even compared to the Nikon family.
If you understood more, you would know what diffraction is, and how this affects sharpness. You cannot say "Comparing with the Nikon Family, I regard the lens is not as sharp enough" when ANY lens, even the Nikon G (which are not as sharp as the Sony Carl Zeiss lenses) would soften and suffer diffraction when shooting at f/28.