Consistently playing moves below my potential Basically: I make blunders. The most disgusting, reprehensible, indefensible mistakes that I would've discarded had even one neuron in my brain noticed it before I moved. I know everyone experiences them to a greater frequency than they would prefer. But I can say that they are without a doubt the number one deficiency in my play, and also the number one thing detracting from my enjoyment of the game.

I play mostly online, and am around 1900-2000 on lichess after several on-and-off years of chess. Up until recently I have always played with long time controls (~30 minutes with increment), although I have started experimenting with blitz  in an attempt to remedy my tendency to take an unreasonably long time studying certain positions and still making outright blunders; this hasn't helped. Regardless of the time controls, I still always feel myself at an imbalance compared to my opponent because while I may have slightly superior technique that'll allow me to chisel fractions of a pawn of an advantage each move, I am more prone to naked blunders throwing the whole game away in one move (and unless it's blitz, I have little hope of recovering since my opponent can play consistently).

I really don't think this is just in my head. I've tried "slowing down" (even though my calculation/processing style is already very laid back), I've tried "checklisting" for blunders (but the checklisting just becomes a hollow routine that can't even cover all the dynamic ways I can screw up), I've tried just waiting throughout months of playing but I don't feel I'm adapting. I love studying and practicing the more refined aspects of chess, and I feel like I'm capable of it the vast majority of the time, but I'm really only as strong as the weakest link in my performance. Has anybody else that's felt the same as me managed to improve their recurrent blindness?
I don't know that I have any good answers for your question, but I wanted to commiserate with you.  One can learn a new opening variation or master an unfamiliar endgame, but such improvement is only incremental as this new knowledge gets applied only once in a while. 

What you are asking for is to improve something that is central to defining your playing strength.  If we could all have perfect tactical vision, we would be as strong as computer engines.  

I'm also relatively weak in this area, but recently I was able to go from uscf 2200 to 2300+.  One thing I tried to do was consider additional candidate moves in my thought process to better avoid tactical blunders, but I didn't find that helped much.

What did help though a bit was me focusing on bringing harmony to my pieces and position with every move.  What I mean by this is that I made it a priority to make sure that my pieces protected each other instead of being potentially hanging on a limb unprotected.  As GM John Nunn has said, loose pieces drop off.  If you have harmony in your position among your pieces, then there are many fewer opportunities to blunder material because even if you miss something the tactics will tend to work out in your favor more often.