How can I prevent my Apple ID account from being recovered using a trusted phone number?

In the Security section of my Apple ID account page (https://appleid.apple.com/account/manage), I have a “Trusted phone number”. The help text says:

Trusted phone numbers are used [...] to recover your account if you lose access.

However, I don’t want my account to be recoverable using a phone number, because phone numbers aren’t secure.

I have two-factor authentication turned on, and I have several Apple devices that support it. I have my two-factor recovery key saved in a password manager that’s backed up in several places. In short, I’m happy that I’ll be able to recover my account if required, without a trusted phone number.

I’ve seen guides showing how to remove trusted phone numbers when more than one is attached to the account (you click the little cross next to the phone number). But I only have one trusted phone number, and there’s no cross next to it.

How can I turn off the ability to access my Apple ID account 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.