I'm also coming to the conclusion that inert gas hypoxia is going to be my way out. My end has to be, as far as I can make it, painless and polite, meaning a happy exit with minimal inconveniencing of as few as possible other people. Any suitable substance for this seems to be taken off the market, even basic stuff like metal nitrites (as used in curing meat. Try getting hold of a nontrivial amount of this in the UK). I don't have the inclination to follow up Project Peanut; I don't have the background for this.

I fully understand why this has been done, to stop people who only intermittently get the urge, but for someone like me it's a major bloody inconvenience, so inert gas it most likely is.

(The above is not an opener for a discussion, it's an FYI for a different perspective. Please don't respond).

Any thoughts on how to inhale it without risking others finding you in a oxygen deprived setup?

Like a warning sign or some fan kicking in later?

You don't set up such an environment in the first place. Tank to flow limiter to hose to bag.

Maybe it could be done outside with a nasal cannula?

i considered diving equipment, but I never investigated them properly.

I had general anaesthesia for a surgery, and I can remember the falling asleep was only very slightly uncomfortable. so I think it can be done - once I'm under, just cut out something important and throw everything in the furnace.