Do not use your phone as a camera on your bike. Here's a link to Apple's own support site.
Exposing your iPhone to high amplitude vibrations within certain frequency ranges, specifically those generated by high-power motorcycle engines, can degrade the performance of the camera system.
support.apple.com
This is while they still are honoring the warranty because they cannot prove this is what broke it, Apple has a trend of excluding things once they can prove the activity.
"Exposing your iPhone to high amplitude vibrations within certain frequency ranges, specifically those generated by high-power motorcycle engines"
Well, I'm good, my bike doesn't have. a high powered engine. 
In all seriousness, from what I've gathered, this is mainly an issue on high revving sportbikes, not really cruisers....well, except for "shake the bike apart" Harleys perhaps...but despite being a V-Twin, my engine is pretty smooth and not buzzy in the least.
I have an aluminum mount that I use on my bike (directly mounted to handebars) that my phone slides directly into. 10,000km last summer with no adverse effects, so I think I'm good.
I could see the chest strap working. But honestly, If I see someone wearing an entire iPhone attached to the helmet, that just looks silly.
I'm not really worried about what I look like to anyone else honestly. I care about coming back with some cool footage to remember the trip by, so from that perspective IGZF.
Yeah, on the dragon where there's photographers I'll run a few times back and forth with the camera in different places so it doesn't ruin the shots (I usually buy one from Killboy when it's all said and done) and then leave it on there for a few other runs to get my own footage.
As others have said, it's not advisable mounting a phone onto your helmet, especially something as expensive as an iPhone 12. You don't want debris hitting the phone, and losing your phone means losing quite a bit of data, is a huge inconvenience and also you've lost your ability to contact emergency services in case something happens.
Also, to start and stop recording on the phone, you need to tap a soft button somewhere on the screen, which isn't as easy if it's affixed to a helmet mount. Especially if the software button is on the front of the screen which is right up against your helmet or chest mount. Most action cameras have an easy-to-access hard button on the top or side that makes it simple to operate with gloved hands. These action cameras also sound an audio warning to let you know when recording has started and stopped.
The "losing your phone" thing is a risk, yes, but having done a lot of digging around on the topic in the last few days, it seems people who do this use a safety tether in some circumstances....and with a quality mount, it seems to be unnecessary even as nobody reports ever having lost their phone from their (again, quality, purpose built) mount. I do have a spare iPhone that I plan to bring "just in case", but I'm increasingly confident in the setup I'm looking to run with here.
As for starting and stopping - Apple Watch. The apple watch actually has an app that interfaces with the phone and not only provides remote start and stop, but shows an exact 1:1 view of what your iphone is seeing on the screen of the Apple watch, which is actually pretty great for setting up the angle etc even when you can't necessarily see the phone screen itself.
Works for video as well as photos as demonstrated in the video.
I dont think that wind resistance + iphone mount on helmet would be best friends.
I could already feel it with my "go pro size" cam (the drag)
So something with such a big surface id say no to unless you have a gigantic windscreen that keeps you in a bubble while you ride
In the end how much is your iphone worth and if it gets damaged would you be willing to pay that replacement
I do have a pretty good sized clear air bubble I'm riding in.
But yes, wind resistance has come to mind. My thoughts on that:
- I have a pretty good clear air zone around my head for a helmet mount anyways, and I have a flip-up secondary windshield (visible in that above pic) that I can adjust to move the air around my head even more if needed.
- I don't plan on using much (if at all) in any high speed riding anyways.
- I don't plan on using it for hours at a time or anything like that, just for highlights....IE, tail of the dragon and some of the other well known stretches of road. Beyond that, it goes back into my holder on my handlebars.
- In some of the bike mount locations I have in mind using GoPro mounts, wind resistance doesn't matter anyways, and again, speed probably won't be a big factor.
Anyhow, here's the mount that I've opted to go with.
It's a tank (solid aluminum) , and it has all sorts of mounting options. And theres nearly zero percent chance of a phone clamped into it falling out.
I'm working on getting some SmallRig gear (That stuff is amazing, I use some of it for other stuff including my handlebar mount for my phone) to get maximum options for mounting angles and such using just GoPro mounts in various places.
I'll share some results as this project comes along.
