I don’t want my phone paired with my iPad how do I undo this.

Try Settings > your Apple ID > icloud and turn off everything (I have only find my phone enabled). Also, access your iCloud account with a browser online and check to make sure everything is turned off.

Apple thinks everyone wants to sync, so that option is on by default. But it is easy enough to turn it off.

Well, it depends on what the OP wants to do with the iPad, oftentimes they want to let someone else use it and thus stop syncing things between devices.

My second paragraph assumes that that's not the case, in which case one would need to know which things to turn off. Turning off everything iCloud on the iPad would stop most, but not all syncing (e.g. iMessage has its own setting outside of iCloud settings). I think the OP needs to clarify what they need turned off.

Having a second Apple ID is not a good idea, neither is letting someone else use your device. Of course you would need to choose what - if anything - you want to sync and I told the OP where that can be done.

I certainly would not want a separate Apple ID as that would create major confusion with Apple devices.

I have never enabled iCloud to sync anything, have one Apple ID, and have never had a problem.

The most effective way is to get a separate Apple ID and sign into the iPad with that. This is recommended if the iPad will be used by someone other than you.

But if you'll be using the iPad and if you want to share some iCloud things with your iPhone, what are you willing to share between devices? If nothing, then as above get a separate Apple ID.