How can I prevent my Apple ID account from being recovered using a trusted phone number?
As far as I can tell, you can’t.

Aside from the web interface, you can also manage your Apple ID in System Preferences on macOS, and Settings on iOS.

When I try to remove my trusted phone number on macOS (System Preferences > iCloud > Account Details > Security), I’m told that I have to have at least one verified trusted phone number on the Apple ID, even though I have Recovery Key turned on.