Your post brings up a lot of potential issues. That was a heavy set for me, and in my lighter work I don't think I get that kind of knee wobble? for lack of a better term.
I got it here as well in my last meet, from a year ago. This was a 40 lb PR at the time, so it too was pretty darn heavy for me.
I'd describe my stance as medium and toe flare moderate. I used to go wider but cannot generate enough power out of the hole; perhaps I was trying too hard to mimic geared lifters.
Perhaps I just need to get stronger?!
For assistance work, I feel I get a lot out of GHR's, reverse hypers, any hip-hinge exercise, hip presses. I probably need more quad work as we tend to emphasize the posterior chain exercises more.
Also, I tend to have more instability on the way down. Once loaded and tasked with getting out of the hole I always feel more solid.
Any thoughts you have on accessories would be appreciated.