I dont like Apple products in general because of the restrictions but one product I have been using for years is Apple iPad. Other tablets just cant compete with it specially when it comes to longevity. 3 years later, it will still be relevant with latest iPadOS while Android tables with Android 11 will feel ancient when Android 13/14 will come out.
The only thing which you can do after inserting a SIM in an iPad is use mobile data on it. No calling, no SMS, just cellular data. Im not sure if it has changed recently or with introduction of iPadOS.
WhatsApp official app is not available on iPad but you can use other Apps (standalone apps) which use Whatsapp Web version in App form.
iPad vs Android tablet IMO depends strictly on what will you be using it for. For video conferencing, I think iPad should be far better than Android tablets but do keep in mind that you wont be able to use cellular calling and SMS in it.