The pedal interface won't work very well - as it reduces the volume to instrument level, which is considerably slower than line level... which is what the 804 will pump out... and you'd still need some sort of audio interface to connect to the iPhone (see below)
the other massive advantage with the 804 is that you only need a single dumb cable - which is a 3.5mm trrs (both ends) cable which shouldn't cost that much...
Any usb compliant audio interface will also work, whether in rack or out... although you'll only get stereo recording - and you'd need a usb to lightning converter dongle and the relevant usb cable - I use one of these with my es8, when I can find the dongle!!! the dongle is quite heavy though and it's difficult to get the iPhone to stay still on a tripod!
If you really just want stereo out of modular into an iPhone - I'd think the 804 is the simplest choice - if you want to also be able to use it with a computer, then I'd consider an usb interface with more channels (both in and out) and getting the dongle for the iPhone...
"some of the best base-level info to remember can be found in Jim's sigfile" @Lugia
Utility modules are the dull polish that makes the shiny modules actually shine!!!
sound sources < sound modifiers < modulation sources < utilities