Forget the SCUBA mask and regulator setup. Get an old SCBA and mask. You can find them on EBay for cheap. Remove from back harness.
You would need CGA 347 to 1/4" F.NPT thread adapter. You can find them on Amazon even.
All in all. Maybe $150 ignoring the nitrogen. SCBAs almost exclusively use CGA 347 so that they can use oxygen bottles from anywhere incase of emergency.
The SCBAs harness and mask has both the high and low pressure regulators between the two.
I use an old SCOTT 4.5 with E-Z FLO regulator. I have experience in a related field with this equipment and know for a fact that it will work as intended.
Be sure to use thread tape or sealant during assembly to prevent pressure loss.
Disclaimer: Life might be shit right now, or maybe you just like having the option(like me) but please exhaust every other option first. This is an extremely lethal option because it's near impossible to mess up. You will black out within a couple breathes so please be careful. I've tested it on myself extensively. I am not recommending suicide as a solution to your issues in the least, I simply believe that this information should be accessible.
You are not alone, make your voice heard, feeling seen and having your struggle acknowledged can truly help. I'm in a professional residential treatment facility (nice house) and the support is actually making a difference.
Lots of love XOXO
- Mayu