Lovely vista Colin
I see the faint red bands you mentioned. Hopefully you can process these out. Are they also there on the subs and stacked panels? What ISO did you go for? The reason I ask is that co-incidentaly, as some form of reference, I've seen 300s dark frames at ISO3200 from the D810 boosted 5 stops with no patterning/banding issue. You probably used much lower ISO, but ...?
Best
JA