An IMEI is burned into each device's hardware. Every single device has its own IMEI so it doesn't conflict with another device. Absolutely no one else can have the same IMEI as you.
Hacking into the phone itself is nearly impossible. From what you describe, someone knows your Apple ID, but not your password. What they're trying to get into is your account. Or, they know both but are being stopped by 2FA.
Best solution. Create a new email account. Doesn't matter where, Gmail, Hotmail, whatever. Just so it's different from what you're using as your Apple ID now.
Log out of your current account.
From another device, such as a desktop computer, login to your account at:
https://appleid.apple.com
Once in your account, it starts at the Sign-In and Security heading. Click the Apple ID button. Your current ID (email address) will be shown. Enter the new email address.
Click the Change Apple ID button that appears. If you wish, also change the password and 2FA (Account Security) entries. Follow the instructions sent to the new email address to confirm the change.
Log your phone back in to your account with the new ID and password. No one but you will now know what these are and the attacks will stop. Continue getting your normal emails from your previous account so you don't have to give the new one out to anyone. The new email address exists only as your Apple ID.