And none of the states using nitrogen as an execution method have any intention of doing it "properly" (obligatory "death penalty is fundamentally wrong").

There's a video somewhere of a documentarian undergoing oxygen deprivation under medical supervision by pumping nitrogen gas into a sealed room while he attempted to solve a simple geometric puzzle (like one for kids, putting the circle in the circular hole, etc). He felt totally fine the entire time but became euphoric and rapidly declined in cognitive ability - to the point where he could no longer solve the puzzle but was very confident that he was doing very well. When he was supplied with oxygen he reflected on the experience and said he had no idea anything was wrong.

Now I'm not saying there isn't something I'm missing or don't understand regarding suicide/execution by nitrogen, but as far as I understand it, any discomfort occurs after you've lost consciousness.

I have a feeling the backlash when states started considering this as an execution method was intended to paint it as less humane than the 3-drug cocktail to propagandize against the death penalty - knowing that if a more humane method were used, the movement against the death penalty would probably lose some supporters. So, they poisoned the well a couple years ago when this conversation first hit the news.

Now, the real argument against the death penalty is that the state shouldn't have the ability to kill convicts because what is a capital offence can change for arbitrary reasons and the judicial system will always wrongly convict people. But a more visceral argument is that execution is painful and cruel - so take away the pain and you lose the folks you've persuaded using that argument.