When working on a 4K one hour FCPX project should I render processing heavy scenes before adding them to the timeline?

In addition to the advice from Luis, another suggestion is you could also work this scene (or all scenes) in Proxy mode and switch to Original media when you want to export for review. Proxy files will take additional space but maybe not as much as a full ProRes export with an added benefit being that you can change your mind about the green screen and not have to re-render the large ProRes.

NTSC is old analog broadcasting standard. It does not use rec 709.

Do you really need proxies? Depending on how you are creating your proxies, they may end using a lot more space. Since you use Proxy Preferred you can create proxies only for specific clips that may need them, but do they?

I see no problem with that. It makes a lot of sense. If you export this in ProRes and reimport the finished scene, there will be no visual loss.

Is your video so complex that it won't play back smoothly without rendering when your Viewer is set to Better Quality?

If it does play back OK you don't need to render it.