It might be tempting to go for Leviathan since it’s THE cold constellation, but I think you’ll be better served going for order/primordial constellations. Kraken is a must for a two-hander and Amatok will boost your cold damage. Both of those require some Eldritch affinity, for which I’d recommend Spider. If survivability is an issue, Eel Crane, and Solemn Watcher will all help you out, and will open up the Tree of Life which gives a massive hit point bonus and healing ability. Also try to take Viper for the resist reduction.
EDIT: I’m doing a cold 2-handed Blademaster that’s very similar to your build and that’s how I did my devotions.