I use 5.3 gr. of Unique under a 115 gr. plated round nose, but 5.9 gr. should work OK. 1.164" is near max for 9mm Parabellum, so I think your change is a good idea. I load all mine to 1.14". Nominal case length is .754", so I'm not sure it's necessary or a good idea to trim them shorter (I don't) although they may shrink anyway. Good luck.
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