Astronauts in the Gemini and Apollo programs breathed 100 percent oxygen at reduced pressure for up to two weeks

There are no reports of "altered states of consciousness" I know of. There would have been rigorous testing before.

Similarly divers, I've never heard of lack/decreased CO2 causing their hallucinations vs nitrogen narcosis or hydrogen narcosis.

This sounds made up, have they been getting high on their own supply?

Those astronauts maintain normal CO2 levels, and divers can experience increased CO2 buildup if they don't exhale properly, which is the opposite.

It's not made up. Plenty of people here who tried breathwork can attest to its power to bring you in strong trance-like states.

Try it yourself. Ideally in group setting, because doing this on your own (with a YouTube video or whatever) won't give you the same experience at all, and could be dangerous if you take it too far.

Good observation! Lower co2 concentration isn't caused by simply inhaling more oxygen, but rather by blowing off too much co2.

Co2 is produced by the body, and the rate at which it is produced doesn't change much if you breathe pure oxygen. it's how we get rid of the co2 is what is being modulated during breathwork.