I looked at a molt shell for an affected shrimp under a microscope:
https://imgur.com/a/9spPMjj
It was full of these little moving bugs, so I'm inclined to blame my shrimp disease on this. These bugs are maybe 20 micron or less in diameter and can swim around 0.5mm per second or so if they leave the shell. I think this is too large to be a bacteria. They only seem to live for about 12 hours or so, or else maybe they all leave the shell after that time because I can't see them any more. I know this isn't a great image to go on, but to get higher magnification I'd need to resort to better microscopy technique next time I can find a molt shell from an infected shrimp.
I'll either need to find the right med to nuke them, or make the tank conditions more unfavorable for them, or maybe make the shrimp more resistant by feeding some kind of immune boosting foods? I tried adding planaria zero (bettel nut extract) and it didn't seem to affect them after 2 hours. Somebody also suggested fenbendazole, but that seems to be aimed more towards worm-like things.
P.S. Can somebody who is good at forum stuff embed this link so that the video shows up in this post?