How do I create a new Apple ID even though I still have one?

With so much tied to your AppleID, your thought to keep it “private” and “out of general circulation” is sound.

Doing so reduces your “attack surface.”

While still an e-mail address, if it’s an address known only to you, your e-mail provider, and Apple … you’ve largely accomplished your goal.

See:

Change your Apple ID - Apple Support

A (modern) Apple ID is an email address. You can change the email address and therefore the Apple ID.

I would not go so far as to call it “unsafe.”

I would however opine that the fewer entities who know - or can otherwise deduce - that any particular e-mail address IS your AppleID …

… the less prone you are to targeted phishing attacks based on your known “Apple’ness.”

Sort of of security through obscurity.

That said, multiple email address brings it’s own “costs” … the technical ability and personal discipline to securely manage and access multiple accounts is definitely NOT for “everybody.”

And poor security practices spread across multiple e-mail address might actually make matters worse.

There’s no one “silver bullet” for security, it’s all about defensive layers.

As another member here recently posted; security is not a fixed, static defense but rather a dynamic “Arms Race.”