I just purchased a M600 Pro and Zenmuse Z15-5D for use with my 5D Mark III. I'm having trouble finding an app that will trigger the camera on an interval so I can do photogrammetric mapping. I've got the video coming in through DJI Go and Litchi but can't seem to set up the camera interval in either of them.

Sorry I meant "GS Pro and Litchi".

Have a look at Map Pilot from the guys at Maps Made Easy.  I don't think it will trigger your camera stock but they may have a gizmo for doing this.