I never knew that “acute intermittent hypoxia” was a known treatment for depression, but I’ve found both freediving and Wim Hof breathing to be effective at treating my depression- however never the two at the same time as that is extremely dangerous.
I’ve found wim hof no more effective than laughing gas. Very short euphoric high then back to where I was plus a slight headache.
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.