I don't use any sort of phone mount, it sits in the tank bag or a pocket when I'm on the bike. My biggest concern would be the consequence of that set up failing....... A tether would make me much less concerned with the phone parting company at speed.
I hacked an eBay bicycle mount on my bike a few years back (through the the stem).
It's still installed, but I only use it with an older phone for music, I never use my "daily phone" with it. Can't imagine your $500+ phone sailing away after a bumpy ride or doing 100km/h and like, oh, I'll just swing back around and pick it up.
On my BMW F800GT have a mount from Amazon _ claw clamps over bar with a bolt to tighten. Has usual clamps for phone and rubber straps that go over each corner of the phone. Ride thousands of miles with my iPhone 10S. If not using phone for guidance will put a TomTom GPS in it. Never had any problems with it doming loose. I did put some small cable ties on the back side to reenforce the top phone clamp part connection to the base clamp part. Love the look of the quad lock but would be concerned about everyday vibration causing cracking of the mount which might not be detected before it failed. I'll probably work on rigging another mount since have lots of time to think about it now.
LOL worst damage to the phone came when i dropped it from waist heigh to ground on gravel - like jumbo sized gravel. Shattered the back even though it was in an Apple leather case! Used on of my two fixes under AppleCare +. Otherwise would have been $650!!! First iPhone Ive bought on launch day. Next time think will go with less expensive version that has a solid back. Great phone and love it.