I don't know what body/lens combination you are using; but, regardless, you should not need an ND filter for shooting fireworks at all. In general, they are used to reduce light and, if anything, they can add more blur to some parts of the final image, which is not what you need in fireworks. As for aperture: 8-11 is a good and sharp enough range, normally, from my experience and you can work with decent exposure ranges when shooting at night. However; if you choose to work with small apertures, diffraction influence would be negligible in case of fireworks, because most of your frame (field of view) is black anyway.
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