You should be able to use the Syphon output from stage setup to allow OBS to grab the input.
Using OBS is a good choice for big video like this, it allows completely separate cpu/gpu processes to deal with the video recording, which will ensure Isadora provides the best possible playback
Yes, set OBS to record at the speed you set the Isadora framerate at.
Since you are on Mac you could try Syphon recorder, it may be the easiest option. Syphon Recorder | Syphon