Not to step in here and throw things off—what I’ve seen and read in this thread all points to Mareck’s—is there any swelling in the leg joints, especially the hocks? About a month ago I had to put down a last remaining bird with similar symptoms to what I saw in your video. She couldn’t stand without wobbling, but her legs were badly swollen, and when she did stand, she had to use her wings to balance. I never did find out exactly what it was, but I ultimately narrowed it down to Mareck’s or Mycoplasma Synoviae bacteria. If you get a necropsy, you might ask the examiner to look into that as well. The symptoms can appear similar to Mareck’s but some serious bio security could protect the rest of the flock. Good luck. It sucks to watch your birds get sick.