A dive into spectral clouds has to be one of the longest renders to do, but the effect is well worth it.  However, if ships are flying through clouds a lot or if the render time is too long, compositing maybe more practical.

One of the first animation we did with Vue 6 was a fly through from space to a flower on earth.  We did it in 3 segments.  The 1st was from space to the earth cloud.  The 2nd was from a Spectral cloud layer to the flower.  The 3rd was just a short fly through meta-clouds.  It was than edited all together.

Each segment had different render settings.

We did it this way because the render time was too great because of the spectral clouds.  We also had problems with random blue screens popping up suddenly in the clouds.  There were also a lot of other problems with shadows and flickers.

It may be wise to send your scene to a renderfarm and discuss the details with them.