How to enable icloud photos on ipad without syncing the device’s photos

iCloud Photos is a synchronization service. Using iCloud Photos ensures that all the pictures in the Photos Libraries on all your connected devices and at iCloud.com are exactly the same. If you edit a picture on your iPhone the edits show up in the picture on your iPad. That's the whole purpose of iCloud Photos.

You can put pictures in albums to separate them, even using the Hidden album. But they will be in the same albums on all connected devices.

You don't say if you use a Mac, but on a Mac you can have multiple Libraries and only sync one of them. So some pictures can remain in a separate Library on the Mac but not be synced to other devices.