Is there any way to sync wifi between devices?
Wifi credentials are actually not stored very securely at all, (plain text!), but in a system folder, meaning that only a rooted app could access it. Google's backup is your best bet.

It's stored in the data/misc/wifi/wpa_supplicant.conf file, for the record.

If your phone is rooted, there are apps like Wifi Key Recovery  and Wifi Password Recovery that can get you the keys, but I know of no apps that will automatically sync these as well.