I highly recommend the book Breath by James Nestor. The only health/pop sci book I'd recommend. Doing breathwork has increased my HRV by ~10ms and lowered my respiratory rate when I sleep. I no longer snore as a result.
What breath work do you do every week please?
Reason why I'm asking: the book contains many techniques, and I'm curious about what's working best for people.
I've read that book and also Patrick McKeown's 'Oxygen Advantage', which also talks about CO2 tolerance and the benefits of Buteyko.
https://www.amazon.com/Oxygen-Advantage-Scientifically-Breat...
https://www.youtube.com/watch?v=-m8jIhQyiDY
Good stuff honestly, helped me a lot