Syncing Music Library on iPhone 12 to Mac M1 with out destroying Music Library on iPhone

It's never been possible to sync music from iPhone to Mac (it's always been Mac to iPhone) without a third party app like iMazing, unless you're an Apple Music subscriber in which case turning on sync library will sync both ways (although you might need wifi for that).

You don't.

Traditional "manual" synchronization is set up to work in one direction: From a Mac or Windows PC, to an iPhone, iPad, or iPod.

• No copying from one portable device (iPhone, iPad, iPod) to another.
• No copying from a portable device (iPhone, iPad, iPod) to a computer - with an old exception (which might no longer exist) for music purchased from the iTunes Store on the portable device. (These days, I think that you would redownload the purchase on the computer rather than a synchronization transferring it.)
• A portable device can only be synchronized against one computer library at a time. Synchronizing against a second library will always delete all of the music loaded onto the device from the first library.