Assigning hotkeys in ZB is fraught with difficulties. Sometimes a hotkey works once only, other times not at all. There are also problems with interface items that aren’t present when ZB loads, and the Z symmetry button is one of these.
Generally, you can reset a hotkey to no hotkey by using the code:
[ISetHotkey,Draw:DrawSize, ]
but for some reason both UNDO and Z Symmetry resist this, and assigning different hotkeys.
You might try this code though, just in case it works for you:
[If,[ZBrushInfo,2]<10,
[IPress,Tool:PolyMesh3D][IPress,Transform:Edit]
[ISetHotkey,Transform:>Z<, ]
[ISetHotKey,Edit:Tool:Undo,122]
[IUnPress,Transform:Edit][IPress,Tool:SimpleBrush]]
[PD]