Highly recommend reading James Nestor's Breath [1]. There is an incredible body of breathing modalities from ancient to modern. James does an incredible job of presenting a non-biased journey through it with a dose of humility and a childlike sense of awe. I finally got my Wimhof practice on track after it.

[1] https://www.amazon.com/Breath-New-Science-Lost-Art/dp/073521...