I don't know the temp when they were having trouble with high speed charging, but (and I am way out of any area of expertise) it looks like the issue was software related. One question I have is if this was on the most effecient tires (21) or the AT (20). They said in conserve mode they got 330, so that makes me think it is the 21s. I think I'd be fine with the 21s, but the 20s are so cool : ) and a 'free' upgrade for the type of order I have.
I do fully expect slow charge rates above 80% - this is a known quantity in the EV world, and is something I DON'T THINK they can code around for the forseeable future.
The person referenced driving in the mountains and getting 240 miles at 20F without hypermiling, which I was happy about.
If this were me, I would pick a spot on the map to stop for 10 minutes when you are in the best spot of the charging curve (20-80%) and grab miles. Rivian has statements out there that I can't recall but I think it is 120 range in 20 minutes.