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. Is carbon paste generally only for carbon on carbon then? Wouldn't be any benefit here?
It works for metal on carbon too, but it's unlikely to help much in your situation.
I like to torque down the headset preload screw to about 5Nm after securing the stem clamp, it seems to reduce the possibility of the preload screw loosening by precession which in turn stops the stem from walking up the steerer.
Sound like the headsets is settling in, start again and set it up.
Make sure to check there's enough gap between the spacers and top cap.
6Nm sound about right to me, you can always go a little over it.