Syncing with iOS devices (iPad, iPod Toich, and iPhone,) and even the old iPods (all of them) is a one-way street always (with 1 exception, see * below.) Meaning, you can only sync from your Mac, to the device, not from the device to your Mac (*unless you have purchased music on your device tagged with your AppleID, that is not currently in your iTunes Library). Apple chose that way to prevent their users from illegally sharing media with others (although, if I remember correctly, it was actually the record labels like BMG, Sony, etc. that insisted on that feature.)
Just follow my steps. Create a new library, and then download your purchased items from iTunes Store.
Before you plug your iPad into the Mac, disable the auto-sync feature in iTunes first, because by default iTunes will automatically sync with an iOS device that’s connected to it, which is likely why you’re seeing the current behaviour.
Scroll down to Step 5:
https://support.apple.com/kb/ph20343
To remove music from existing iPad ... Scroll down to the turn off syncing section:
https://support.apple.com/HT201253
If that’s too confusing for you, then probably best to just erase everything on the iPad and then set it up as new with the new iTunes Library:
https://support.apple.com/HT201274