Check this to determine if the advantages of the a6400 vs a6300 are worth it to you:
What not many comparison sites mention is that the a6500 body and grip are a bit beefier than the a6300/6400. To me the a6500 has much better handling, and I can immagine that if you're coming from a Nikon DSLR the a6300/6400 may feel a bit tiny in the hands. The a6500 wasn't on your lost though so don't want to complicate things more 😉
The 18-135 lens is considerably sharper than the 16-50 and has a useful range if you do occasional wildlife stuff. Then again, the a6400 has animal eye AF, though I doubt either of these lenses lens will give a small enough depth of field to see whether focus was on the eye or face... Choices...