I've revised my list and you should PRINT a copy of this incase you lose access. * means phone is OFF/Flight Mode.
*1. New SIM card with NEW number.
*2. Wipe the phone back to Factory Settings in Reset.
*3. Create a new gmail.com address on a computer that this person cannot access (different wifi network).
*4. Use a very strong password for the gmail account, and add two-factor authentication.
*5. Create a NEW Apple ID using the NEW gmail account.
6. Put the NEW SIM in the factory restored phone and setup your Apple ID on the phone.
*6B. Enable two-factor authentication for your new Apple ID.
*7. Do NOT download WhatsApp or We Chat.
8. Leave the phone as basic as you can with apps for a few days. See if they can hack you?
9. They may also be getting your MAC address off of your work/home network, I'm assuming you share the same wifi network at work?
10. Consider only using mobile data at work with wifi fully turned off in settings, make sure bluetooth is also fully turned off.
*11. You won't need a new phone if you follow these steps.
12. If you must use WhatsApp and We chat, only install one of them at first with a new account/number.
13. I'm guessing We chat can leak all your info. So use that last if you have to.
14. On WhatsApp and We chat. Do not give permission for the microphone/photos/camera. (Turn on/off if you have to!)
*15. Use 'Flight Mode' whenever you're not using your phone (Wifi and mobile data OFF).
*16. Keep a record of all your other Apple IDs (There's a limit how many you can make) and passwords.
*17. Change all the passwords to your old Apple IDs, keep records. (Keep records of everything).
*18. Old email addresses too, change passwords, keep records of login name/password.
*19. Write it all on paper if you have to but encrypt the passwords on paper by reversing the order of the letters or something else CLEVER.