I’m not an MDM expert but it’s unlikely they can read your whatsapp and sms messages (but not impossible I guess). Be surprised also if they can delete your whatsapp messages, and very unlikely they can delete “just” company related ones. Would be surprised if Whatsapp an authorised comms channel for company business tbh.
Usually you get access to a sort of company app store where you can download MDM controlled apps. They’re the ones that your company has more control over.
Having said that, putting MDM on your personal phone is a matter of how much you trust your company and the way it is run. Also how likely you are to be called up on some sort of disciplinary.
Again, I am no kind of MDM expert.