You can still use a separate AppleID on the iPad but it cannot use your iPhone number for FaceTime nor iMessage and it cannot share your iPhone’s SMS/MMS messages. There is no way around that. Nor will that iPad be able to handle calls as that too requires devices to be using the same AppleID.
Facetime and iMessage can be used on the iPad using the new iCloud email address you’ll create on it with the new AppleID. And they can use any other additional emails you associated with that AppleID as reachable at email addresses.
But if you wanted to use iOS/iPadOS continuity to share the iPhone’s cell number and cellular messages and calls with the iPad, there is no way to do that and use separate AppleIDs on the two devices. All of those features of continuity only work when one AppleID is used with iCloud on the connected devices.
If you used your personal AppleID on the iPad, you could go into iCloud settings on the iPad and turn off most of the data sharing services (turn off iCloud photos if you use that on your iPhone, turn off iCloud mail if you don’t want personal mail on your iPad, turn off contacts, notes, iCloud Drive, etc). That would keep most of your personal data off the iPad but would let you use continuity to make and receive calls and SMS/MMS MESSAGES to your iPhone number on the iPad and use your cellular number with iMessage and FaceTime. You’d have your work email for emails on the iPad but your iCloud personal email does not need to be enabled on the iPad.
Plus you’d then have two trusted devices for appleid 2FA codes in case something happened to the iPhone.
If you backup the iPad to your iCloud, it’s backup will be a separate file from your iPhone (iCloud backups are based on the device name, so each uniquely named device has its own backup file) so there is no mixing of data there.