I’ve always used 6mm microbore. Light and flexible.
I also use slick-connect system. It worked great . Best connection for joining reel hose to pole hose imo. It saves the hose catching on objects so much in a days work. (Don’t bother if using hot though, it cracks with the heat over short time).Especially good with the Exceed innovation setup., Univalve.
You can use the red hose too, it'll be fine with cold and if you get the immersion fitted. I used thermobore for quite a few years and I was only using the immersion. It lasted really well.