Hi @Adam and welcome to the community!
The ground concept that can cause this is that Dorico can write from pppppp to ffffff, and all the settings are proportional to this. So I experienced that you have to reduce the total range, so that mostly the middle dynamics, between pp and ff are mostly used and distributed.
see this also:
especially the referred link to the Dorico_5.0.10_Version_History.pdf
I tried out with Iconica Sketch and look out for settings that give you a more variable dynamics between pp and ff (what is normally used). see my example at the end of this post*.
You have to go in the opposite direction for the minimum and maximum dynamic levels (I adjusted this at +2 and -2) and augment the dynamic curve (I adjusted it at 5) [EDIT: this last setting can be also putted at 1, as the default override in the Iconica Sketch expression map, the main difference will be given by the maximum and minimum dynamic levels]:
In the expression map…
… I deactivated this option…
…to experiment, so that I can change the values only in the Playback Options window, if you then find your value, you can set it back in the expression map for Iconica Sketch, as override [as I said the default is 1], and put back the default value of 2,5 in the Playback Options window.
Not sure how the minimum and maximum dynamic level settings influence other playback templates and libraries…, but you can experiment.
With this setup you have a nice change between pp and ff, and Dorico uses the whole range of Iconica Sketch dynamic. You can of course experiment with different settings but I think it could be a starting point. And when you change a setting, try to playback with all the dynamics having the Halion vst windows in sight, so you see how the (CC1) signal ist controlling the dynamic of Iconica:
*) If you want to see and hear, I made an example with this settings, try to change from pp to ff the initial dynamic of Horn 1 and you will hear what I mean:
iconica sketch dynamics curve.dorico (2.0 MB)