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.
https://www.amazon.com/Oxygen-Advantage-Scientifically-Breat...
https://www.youtube.com/watch?v=-m8jIhQyiDY
Good stuff honestly, helped me a lot
Reason why I'm asking: the book contains many techniques, and I'm curious about what's working best for people.