Hi there,
Apologies if this has been suggested to you already.
(I read your query and thought I'd pitch in quickly to assist, but I've not looked at any of the other responses you may have received)
Ok, I was in the same spot after switching from everything Android to iPhone and iPad a year ago.
The only solution I found to successfully duplicate your iPhone WhatsApp account, to a functional client on the iPad was via WhatsApp Web.
Open your WhatsApp app on your iPhone and open the menu using the 3 little dots.
Go to SETTINGS and select LINKED DEVICES.
Now tap LINK A DEVICE.
Open SAFARI on your iPad and go to web.whatsapp.com .
The page will display a QR CODE here.
Use your iPhone to scan this code to authenticate and LINK THE DEVICES.
>>>PS: This works in just about any browser on any device instead of an iPad, including Windows, iMac, etc.<<<
Hope this helps in some way.
Cheers,
Yusuf