Personally I have NEVER found ANY recording made by ANY DSLR camera any use other than for syncing up purposes. To have to use it in a mix is highly regrettable.
You will find that the noise floor on these things is horrible, and the processing, AGCs and clipping even worse. Increasing the gain will just, as you have found increase the hiss massively.
Izotope rx can “learn” the hiss out of a mix, but yooull find that a lot of the spoken word that you want to keep will also have frequencies in the hiss area, so the benefits might be limited. If you over push isotope it will go “watery”
I dont think it will save your bacon.
Id also suggest that you take a good look at your setup. Zooms are fine, but what happens when they fail??? Unattended recordings are likely to let you down. batteries run out, things get moved knocked, recording levels arent right (and the auto level in a zoom is completely useless and DANGEROUS) Thats where you are now.
When I do weddings I use radio mics back to my camera, and a proper video camera not a stills camera.
Radio on the Groom, radio on the readings lectern, radio on the vicar as a minimum. I have proper split channel recording, metering and headphones on everything.
AND most importantly of all, ALWAYS have a wired mic nearby. I use a rifle mic on a floor stand only feet from the people. Thats the safetyy and i would NOT be without it.
Good luck but this really emphasises the words of my mentor when I was a tv trainee. Get it right on location. Therell still be plenty to fix in post. If you drop a ming vase down the stairs, no amount of superglue will get it back to its original value.
Peter
Post Production Dubbing Mixer