How to re-add a stolen iPhone to iCloud for tracking

You can't. Not unless you recover physical possession of the device.

Removing the phone from the list of devices associated with your Apple Account cleared Activation Lock, which would have allowed the thief, or a fence or "customer", to associate a new Apple Account with your phone - and "make your phone their own."

My understanding is that Find My might continue to show the phone for up to 30 days after the removal of the phone from the list of devices associated with your account.

I'm guessing that any such tracking would stop the moment that the thief "made your phone their own."