you didn’t mention what type of piping

I didn’t build it. I can see that the entry fitting & 2nd fitting is PVC, but that’s all I can see without a snake cam for wet environments. The behavior with the snake clearing the line temporarily, then the line being unclear after pulling it somewhat suggests that maybe I have a shitty goffered (accordian) pipe. Maybe it’s getting kinked or folded somewhere.

I am wholly willing to bet that the sulfuric acid didn’t fix your problem but rather put a hole in the pipe itself above the blockage. I guarantee that you are now draining the contents of that drain directly into either a wall or some unseen cavity and are not actually draining out of your home/apartment/wherever.

That was my worry indeed. But I figured my next step is to replace the pipe anyway, so it was time to test the nuclear option. The main sewer line in the basement is accessible just before going out to the street. I could clearly hear the water running through the pipe when the kitchen sink is draining. But I can’t quite judge if the sound is gushing to the extent that it should be. So jury is out on this.