I was trying to figure out how to change the modifiers that allow changing brush size/foreground color behave, and have been led to the Canvas Input Settings menu. However, documentation on this area is very thin and there is some terminology that unclear.

Specifically what are Primary Mode and Secondary Mode?

While it doesn't appear to be possible now, is/can there be a way to configure the control modifier to adjust opacity (the way the shift affects brush size)? Even better would be a way to separate x & motions holding a click to adjust a slider; IE shift would be able to control size horizontally and opacity vertically (that would make artists switching from Photoshop to Krita love it even more).