the dynamic curve power applies exclusively to the primary dynamic controller as far as I’m aware (if wrong then someone please correct) which is not necessarily velocity. It is, in effect, Dorico’s replacement for the Sibelius function of specifying fixed values for written dynamics and there are pros and cons to both methods.
The other thing you can do is to scale the controllers in the EM so they are not in lock step. Velocity will usually have a range from silent to very loud whereas timbre dynamics (typically CC1) may be much more restricted depending on the library. If the Garritan Big Band library uses sharp attack with velocity and volume with CC1 as you suggest, then that is the reverse of the most common practice. Setting both to the same value can make for too extreme contrasts so it’s worth experimenting a bit.
There will inevitably remain some situations where there is either too much or too little attack for the corresponding volume. Then you’ll need to drag the velocity bars up and down or draw in an individual point or line in the controller lane as appropriate.
To me it would be great if there could be individual dynamic curve settings for the two different controllers (and within them greater configurability). This could help reduce the need for individual adjustments.