I regularly monitor my O2 saturation (via an OTC pulse-ox) when flying unpressurized airplanes.

It’s extremely easy to raise my O2 sats at altitude by conscious deep-breathing, but IME, it’s incredibly difficult to lower them near sea-level by any amount of conscious breathing changes. I’m curious to try the Wim-Hof methods on my pulse ox and I’ll do that, but I fully expect to see consistent 98-99% O2 sats here at 10 meters above sea level with anything other than changing the air composition to reduce partial pressure of O2.

From my understanding, the deep breathing doesn't particularly raise O2 (which is already 97-99%) but purges CO2, which then lets you hold your breath (empty lungs) for 2-3 minutes. In this period, my O2 (on my finger) drops to 85% before I feel a need to breathe in. It is important to 'replenish' the CO2 between deep breathing, otherwise I get very loopy.

Yep, and it certainly helps. Also Dr. Joe Dispenza's Mind Body Connection. I think a lot comes from stress, anxiety and dealing with your body with a wrong mindset.

But it doesn't explain all the issues away.