After testing and searching paid and free apps for hours. I found a free app that does this. TrueVisage. Tested with iPhone 8, connected to my MacBook with USB-C cable and recording into Quicktime. It has a minimal interface that disappears after a few secs. Leaving beautiful, HQ, controls-free, watermark-free views of front or rear camera. Also has a handy flip function. Winner!
This allows me to use my iphone like an external web camera, connected via USB-C, which looks much better than the icam native to my MacBook Pro.
Note: Notifications from other apps may still come on screen and show in the view. Disable notifications before going live.