If you do not want to wait for the backup of your Iphone to your computer, when you just want to sync your music library on Itunes, this could be the solution. Open up a terminal on your Mac and type in:
defaults write com.apple.iTunes DeviceBackupsDisabled -bool true
This will skip the backup process and continue with syncing.
Original post was found here: https://discussions.apple.com/thread/8069549
If you want to revert this change, you have to alter the boolean value to false. Open up a terminal and type in:
defaults write com.apple.iTunes DeviceBackupsDisabled -bool false
Do not forget to backup manually from time to time.