Gunther, would it help to render my sunrise in 4K (instead of 1080) before adding it to my Final Cut Express timeline to output a master Quicktime movie (without compression), and then compress from there?

The resolution is not correlated with the bit rate - so that shouldn't matter.
It makes sense to export in 4K if you want to do some cropping in post, not to get better quality.

If you have banding, you should try using higher quality settings, go ProRes and/or use the LRT Plus Motion Blur filter.