Taking pics of dancers in a ballroom

First of all, the picture you linked to came out very well. That lighting certainly poses some challenges for focusing. I have taken shots in that kind of lighting with AF on my 7DII and 35 F2 IS without any problems, but that camera's AF system is optimized for low light. I have also managed decently with AF on my original M using the 22 pancake in that kind of light. The 50 II is notorious for slow and bad AF. If you want to use a 50, I would recommend the 50 F1.8 STM, not the old, and also not great for AF, F1.4 version. I would also highly recommend the 35 F2 IS for those conditions. You might benefit from the somewhat wider angle. I just recently got the M6, and can say that the AF is very impressive. In low light, with a fast lens, like the 50 STM or the 22, it works very well. I used it with both of those lenses last weekend at a dimly lit theatrical fundraiser, and the AF was fine. I got some shots of an Irish stepdancer using servo with the 50, and the focus followed pretty well. The lighting prevented me from using shutter speeds high enough to completely freeze him, but the blurred feet combined with sharper torso worked pretty well.

I would say that the M6, combined with either 50 STM or 35 IS, would cope perfectly well with AF for what you want. The 7DII would be even better. I haven't used the latest Rebels, but I would think that at least the center point would also cope well in those conditions. The 85 F1.8 is a pretty decent lens for fast AF. I've owned one for the last 12 years, and use it periodically. I haven't used it with my M6 yet, but I used it for portraits with my M, and the AF was fine. I would think it would be a bit restrictive for the kind of conditions you're talking about, though. Depending on how close you are to the dancers, you might also think about the 22 pancake.