I think Derek Humphrey came first and promoted the exit bag. Not sure who came up with veterinary N. I'd recommend doing some research if you're really interested. I do know that Nitschke popularized suicide tourism such as travel to Tijuana, and even used to go with groups, and as a result, eventually there was a lockdown of N being sold in vet pharmacies in many countries without a prescription, including Mexico. I've read that Nitschke came up with the SN regimen, though SN had been used in suicides before, but was not popular knowledge.
Nitschke is the "big voice" in euthanasia right now, but Derek Humphrey is still around, and there's the author of Five Final Acts, and there are organizations affiliated with both. And of course there was Kevorkian, who worked alone but was also political like Nitschke, and he did design a device but the meds are hard to get.