@eva

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

I solved it for now

Hi Izzy support,

Found a possible bug in 4.5.1 "when combining video" scaling settings

Am able to exit settings when select "Scale To Largest Image (Slower)" and "Scale To Smallest Image (Faster)" but if select "Scale To Default Resolution (Legacy)" am not able to exit settings and have to force quit Isadora

Isn't a biggy since i never use this option but thought i'd raise it

Great product, you've got a customer for life, Mr J