Thanks, everyone.
I can see that some iOS rigs could be a bit of a hassle to work with. It would be device and interface dependent, but it might be difficult to neatly dress some of the leads, especially if some ports and connectors are on the 'wrong' side, plus additional batteries are just another thing to worry about. Kindms' configuration with the Sonosax's internal battery and no requirement for a CCK would keep things much tidier. Perhaps it would be possible to mount everything on some sort of board.
I appreciate the comments about the old iPod Touch. I just thought that I could pick one up fairly inexpensively for testing, and the Apogee app is listed as being compatible with older iOS 10 devices. However, that's not the same for all field recorder apps, and I know that Apple can move the goalposts at any time, which could give rise to compatibility issues.
I need to have another think about how I would use the recorder. My static recording rig is a decent one, with a good quality interface and external preamp etc, so I'd be reluctant to give up much in terms of sound quality, although perhaps any differences wouldn't be particularly noticeable.
As with anything, some problems in deciding come after reading reviews! I was quite interested in the Centrance Mixerface R4R\D as a standalone recorder, but it didn't fare very well in ASR testing! However, even the MixPre didn't survive unscathed.