Quote: ...how do you adjust pen pressure for size/opacity at the same time..
Seems you already did this, make a new dynamic, and using Opacity + Size
(1) Is what you get when opening the dynamics settings from the Brushes Tool Options, greyed out because it in the Gimp system folders.
(2) Is when opening the Dynamics dock Windows -> Dockable Dialogues -> Paint Dynamics where you can make a new Editable dynamic.
If the new opacity / size is not working then look at the Gimp Pressure Curve Edit -> Input Devices -> (your device) -> Pressure I find this takes over from the tablet settings. Does the default Pressure-Opacity dynamic work.
Even if you get it working I would not expect anything fantastic in the size versus opacity range.
Quote:I don't see any way to edit brushes (new ones) ..
Editing a brush is not the same as adjusting a dynamic, What sort of brush do you need to edit, A parametric (.vbr ) or a bitmap (.gbr) there are also gimp image hose (gih) and PS brushes .abr ?