Not the greatest video sorry. I think you can tell the base is stable but the chassis moves.
Yes. I have.
All hex bolts are tight.
Getting the same flex in the body with light and very dark roasts.
I'm not seeing "flex" it just looks like wiggling to me. The whole machine is wiggling a touch, maybe a washer to make a better contact with your bolts. Also, on many occasions the towel is obviously jerking and boopping alongside it, so it appears to be movement from both the grinder body wiggling and the entire base lifting/wiggling, but definitely not the shaft "bending" or flexing.