I had something else in mind that would stick to the "you're able to select your targets to have more control over your reactions".
First one : when you give the die, you also choose against which target it will trigger. I guess it would be complicated, especially in multiplayer. => It would mean that the caster will have to "recast" each time the player with the die wants to change the target.
Second : Rather than "giving the character" on/off toggles, it gives him an activable icon he can also toggle on/off (right click to toggle, left click to activate). When he click the new skills on his hotbar, he can select "manual target" or "auto target". => So you give your die just as now but rather than only being able to turn your toggles ON/OFF, you can manually select the targets that will trigger them.
The one thing that bothers me slightly about your solution, even if it is somewhat elegant, is that the ability to set specific targets for activation basically pushes it into the realm of metagame knowledge. I mean, my proposed solution would kind of do that too, but there's a slight difference between generalized conditions that would trigger equally for any enemy able to meet said conditions, and outright targeted reactions.
But at the end of the day, it's only a minor complaint, so eh. Still better than whatever the hell we have right now.