iPhone reset without Apple ID

Few months ago I updated my Apple ID, chaining the email address tied to it from X to Z.

I own two iPhone,

- in my iPhone 12 mini, which I regularly use, no issue

- in my iPhone 7, which I don't use regularly, I forgot to logout before the change

Now I need to reset my old iPhone 7, but I can't as it is asking me to insert the password of my old Apple ID which is not active anymore.

If I try to reset the password, it doesn't work either (which I think is normal as that account doesn't exist anymore).

I also tried to change my Apple ID to, again, my previous email, but I received back the following message: ‘This email is not available. Choose a different one’

Can you please help me with this issue?

Thank you