It's not exactly a minimal pair, but think of шок in Он испытал настоящий шок (He experienced a real shock) and щек in У него лицо без щёк (He has got a face without cheeks).
@pixel That is exactly what a Muscovite told me.
I have this page here. I can hear шоколад has a sharper, almost whistle sound and щётка is softer like schön, but I don't reproduce the sound of ш consistently or well, so I've kind of given up.
However, no one will ever have to put up with me, because my goal is really just to be able to read and listen a little.
I only know 2 ppl who have russian, but they don't like it (they are older poles).
Good luck with your journey! Feel free to ask questions if you have any.
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