I'm in your shoes too, I'm using an iphone 7 but fancy the improve picture quality of the 12 pro for sure....I actually did buy an 11 pro from Music magpie but it was scratched too badly so sent it back (£589).
On the trail though, even though that iphone 12 pro would be great I very much doubt I'd take it, it would be pretty daft really unless your very risk averse but even then shit happens; you wouldn't get much sympathy for breaking a 1k phone mountain biking!
I bought an apple 6 watch, sent that back as it was a delicate little toy that would shatter the second it got any interaction with anything other than mud and the battery is a joke. The garmin edge Ive tried, its like using a gps on a spectrum, beep beep beep.....collision detection was nice though I think, a tesla collision avoidance system would be nicer!

, doesn't take photos also! Thats left me with my iphone 7 for better apps on nav/trails and takes a picture; but the battery sucks, the new 12 is worse than the 11 and is 25% less than standard android phones even.
Not sure how you feel but what if you bought a bike specific phone and run a cheap contract on it? There loads of great android (I know im swearing) devices with superior battery and cameras for half the price of iphones (Maybe not as great as the iphone 12, but certainly close!).
For quick access you could stick on your bars (inside a shock case) like so
https://amzn.to/3662Bls (maybe get one that tightens with an allen key) then you can just twist off and shoot. If it breaks great, its way cheaper to replace!
There is an alternative to this too, you could buy a gopro hero 9 which is on a black friday deal until end of this month I think
Black Friday HERO9 Black Camera Deals & Bundles | GoPro with this you wouldnt need to snap anything as you could just save a frame out of the video you produce; set to 4k the stills can be quality! It also does have a snap mode, you could mount on your chest, chin, helmet top/side or handle bars.....the ultra wide will catch way more than you expect so no worries about missing any of the surroundings.
My current setup is Garmin 6 fenix pro solar (£370 refurb) + iphone 7 (£100) + gopro hero 9 (£329)....tbh its working alright, I've not used the iphone for photos since using the gopro 9. I would advise a shell on the watch though.
Despite the temptation, vibrations risks and mtb activity isn't the best match for the 12 pro.....I've spent £800 and at least 2 of those devices are built for things like mtb, the phone is just there for phone calls or trail detail from trailforks which the fenix cant give me at times