I don't think breathing techniques are going to help very much when these are the effects reported by Florian Dagoury, who's officially held his breath for 10 minutes and 30 seconds.

From the linked article-

Florian Dagoury, the current world's top static breath-hold free diver - he officially held his breath for 10 minutes and 30 seconds - has been diagnosed with myocarditis and pericarditis 40 days after his second dose with the Pfizer vaccine. The free diver shared his adverse reaction on Instagram:

 "After my 2nd dose I noticed that my heart rate was way higher than normal and my breath hold capacities went down significantly. During sleep I’m at 65-70bpm instead of 37-45bpm. During the day I’m now always over 100bpm instead of 65bpm, even when I sit down and relax. Once I even reach 177bpm while having dinner with friends !!!! 40days after 2nd jab, I had no progress so I went to see another cardiologist and got diagnosed with Myocarditis, Pericarditis and Trivial Mitral regurgitation! I’m now struggling to reach 8min breath hold, 150m down and I even have a strong urge to breath doing 40m dives. 30% decrease on my diving performance roughly."

Breathing techniques may help some, but I'm sure this guy knows every trick in the book.

He's now struggling to reach 8 minute breath holds which means he's taken a ~27% performance reduction he's attributing fully to the mRNA vaccine.