Can you not plug directly into the phone, or are you trying to avoid a dongle adapter thingy getting in the way? The transmitters are capable of recording audio by themselves so you could do that and you could get reasonably decent quality with a well placed lav mic plus post processing the 32-bit audio (I guess, I haven't worked with 32-bit yet). Basically all you're getting out of the iPad is monitoring ability, and it's possible the iPad might degrade the quality? I would definitely run some comparison tests with the onboard vs running it through the iPad.
Yeah transmitting to iPhone would mean adding a usb hub and power pack to the rig and is just one more thing for me to monitor/adjust vs having an assistant monitor sound and adjust levels. Will definitely do some field tests to check for any degradation when recording to iPad. The Rode app is designed for such a setup so not sure why the iPad would cause any interference but worth checking.