This topic is a bit older, but still interesting to some, I guess.
I am using my iphone as an audio recording device too, with OKM mics. There are two devices available: Soundman modded the Zoom iQ6, took the capsules out and put a 3.5 Jack in. He sells them for 160€. Recently Rode released the SC6-L Lightning Connector with 2x TRRS input, for 3.5 Mics you need an additional cable to split the signal. Soundman offers these in bundle (also with mics), the Rode device alone is about 70-80€ (still cable needed). Both can be used with all external mics, either you need a small cable for one or the other occasion (splitting the 3.5 for Rode or combining for the 3.5 Jack of the zoom).
the Zoom device has a physical level control (Rock shows are suitable with Level 2-4), the Rode doesn't. The iOS App Voice Record Pro has all useful features for free including a Input Level Gain. (As read on the internet, the VRP App is not as useful for Android). You can combine the Zoom with VRP too.
Attention: you might come to conclusion, you would need the Rode Reporter App. Actually, with this app you can set up the 2 channels separately or combined, which is used by Voice Record Pro (VRP cannot combine channels itself, without Rode App it can only record separate channels) and a basic Input Level (Low, Medium, High) that will also be accepted by VRP. BUT: as long as you have the Rode on the phone and once made settings in it, it brings distortion to the recording (crackling that lies "above" the sound and is not on silent parts so actually NOT removable) when using the Rode device. Delete it from SETTINGS if you might have used it, then all settings are gone too and the recordings are clean again.
The Zoom is a bit thick to carry because of its capsule structure, that remains. I usually put it in a small back of throat drops. The Rode is very flat, the cable is a bit longer, but can be hidden quite well in flat back pockets of a small bag or something. I recorded loud rock shows without any batterybox/preamp and they are really nice, so that is not really needed.
Hope this info might help one or the other on here.
For video & photo I use Panasonic TZ (ZS) cams.