I fixed it!! It took a better part of the day to tear it down and build it back up but at least now I can print without being ashamed of the quality.
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Left is before, right is after. Same gcode.
I did a complete realignment of the bearing blocks and bed mounts to make sure they were lined up. I also tightened up the V wheel tensioners on all 4 corners and ensured that the bed raised and lowered smoothly and evenly by hand. Plus I replaced the 40T lead screw and drive pulleys with new ones from OpenBuilds. This time using a 20T drive pulley giving me a 2:1 gear ratio. The belt was also replaced. The cheap toothless flanged idler pulleys on the belt tensioner were replaced with flanged 605 bearings which ran much smoother and didn't rub the belt like the old idler did.
I don't know if it was just one or all of those things, but the banding is completely gone. I suspect that the biggest issue was the pulleys. There was visible wobble and movement with the old ones. That's all gone now. It's hard to put a finger on it, but visually the quality of the belt movement just looks better than before.
Thanks everyone for the suggestions.