Open Settings on the iPhone
Tap your Apple ID name at the top
Tap Devices
Select the iPad
Tap Remove from Account
On the iPad, open Settings
Tap your Apple ID name at the top
Scroll down and tap Sign Out
Enter the Apple ID password if prompted
Turn off AirDrop on both devices
Turn off Handoff on both devices
Turn off iCloud syncing for apps you do not want shared
Turn off Messages in iCloud if needed
Turn off FaceTime on one device if you want separate calling
Turn off iPhone Cellular Calls on the iPad if enabled
Remove the iPad from Find My if you want full separation
Restart both devices after making changes