I might be doing something wrong, but when I tried to set BlackMagic Camera to ProRes Log 4K60 it had no problem recording it. I was even able to apply different free LUT files and export it to photos. It is 1.6Gb for 8 seconds, but did not require an external disk. [1]

[1] https://imgur.com/a/9vUdPcg

Edit: iPhone 15 Pro 128G