Unsyncing

As both your iPad and iPhone are signed-in to iCloud with the same AppleID, you have full control over the data and services that you wish to synchronise via iCloud…

You simply enable or disable the services and Apps that you would like to synchronise between your devices:

Settings > [Your Name / AppleID] > iCloud

Settings > [Your Name / AppleID] > iCloud > [Apps Using iCloud] Show All Apps

The iCloud User Guide provides comprehensive information:

iCloud User Guide - Apple Support

What is it that you wish to achieve by doing this because you can’t really “unsync” those devices are you trying to make it so that you can not use iMessage on your iPad?