As most have said, doubling up compression won’t usually get you much.

However, video compression is usually designed to facilitate performance of sequential reads because videos are typically played beginning to end, so theoretically there may be ways to compress them more if you’re willing to make sacrifices there.

I doubt RAR is the way to do it, though. It just hasn’t been designed for this kind of data.

Maybe there’s a video compression format out there designed specifically for archival storage, but I’m not aware of it.

ISO won’t get you any further compression, that’s for sure.

You could certainly test this out yourself and let us know if you get any space savings.