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Just a thought and I haven't done anything with it for a number of years but since the Quick Access and Libraries items in the left pane in File Explorer are more or less shortcuts to the folders in C:\Users\USERNAMEHERE it may be necessary to customize folder icons there first.
Hello @Nikbawker,
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Does this help? => Change Icons of Folders in This PC in Windows 10
That's the thread where I originally posted this one of the admins must have moved it. Going to have to look around and see for myself I guess which will work, that post is the most appropriate post though which is why I posted this in there as a comment initially before it got moved.
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I actually did test that prior to posting it only changes it in the main window but not the actual side quick access panel. Unless it's a sizing issue maybe there is a way to apply a small icon to them that way don't know for sure yet I'll try doing some smaller icons on them see what happens.