Completely anecdotal but I've found Ujjayi Breathing from yoga to have an enormously calming effect.

Long exhales in general with a short pause at the end, before inhaling, are also incredibly useful. I try to do this as often as I need.

Interesting… I will look into that. Part of Wim Hofs teachings include breathing with an extremely long exhale while humming which sounds related. I’ve heard of research showing that any type of breathing with longer exhales than inhales activates a parasympathetic response, calming you down.