How can I be sure that all my data was deleted from my stolen iPhone?

Here is what to do: If your iPhone or iPad was stolen - Apple Support

Your data is protected by your passcode and by biometrics, and by Stolen Device Protection if that was enabled.

The erasure cannot be cancelled once completed, so apparently the command has not yet reached the iPhone.

The iPhone may well stay offline permanently, too.

You may get a call from “Apple” or “police” reporting the iPhone found, and requesting you prove your identity by providing your Apple Account credentials. Don’t. That’s the thieves.

emergency_pleasehelp wrote:

The only command I can send right now is Cancel the erase or Unlock this device written in red color. Is there anything else i could do possibly?

You could ask your phone company (carrier) to blacklist the phone. If they do, no carrier who honors the list will ever provide cellular voice, message, or data service to that phone ever again.

Not that the criminals are going to be getting into the phone to use any of those things if it was passcode-locked with a passcode that the criminals do not know and cannot easily guess (i.e., not something like "123456").

Blacklisting may interfere with the phone's ability to learn of "Mark as Lost" and remote "Erase" requests - so you may want to put it off for a few days to give the phone a chance to learn the "Erase" request. (Even if the phone never learns of the request, there's only 1 in 100,000 chance that the criminals could guess the correct six-digit passcode before the phone locks them out, and goes into a mode where you have to wipe it (and also satisfy the Activation Lock) to get back in.)

Do not remove your phone from the list of devices associated with your Apple Account, unless you want to give the criminals a present.

Removing your phone from your list of devices will clear Activation Lock, allowing the criminals to reset your phone and "make your phone their own", to have a working phone to use for themselves or to sell to others. (If you make the mistake of removing the phone, it may still be visible in Find My for up to 30 days, but you will not have any way to cancel the action.)

What is done is done, you have followed the steps per Apple, your iPhone is not coming back, your passcode choices are what they are, as are your backup choices, start looking at the replacement.