I've thought long and hard about this also. I would suggest you checkout Hitcase, they make a chest mount and a handlebar mount. Also for the Iphone try the Vidometer app, it gives the ability to do overlays. I have not tried any of this stuff but this is what I have found so far that seems to fit the bill in my searches. I'm struggling on trying to figure out what to do with my aging Gopro Hero 6 and 5 session. Do I go for the Hero 9 or the Insta 360 one R. Or do I do what you are thinking, I always have my phone with me anyway to track the ride. I might end up doing both. Not sure I would want to use the Hitcase option for a helmet cam. But having the option to do a chest or handlebar mount sounds like a viable option when I don't want to carry all the extra gear with me.
The handlebar mount has appeal just as a mount to hold the cell phone for navigation and then switch to video if needed. It almost looks the same as a Gopro style connection so I wonder if it is compatible. I would think a better handlebar mount would be available, the one they offer seems to use wire ties but not sure. The stuff is expensive and if you upgade your phone then you may need a new case as well, money that could go to another Gopro.
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