I think I have a better solution.
So, you want to use smartcasting primarily and be able to view the range of spells when need be, right?
If so, why not place smartcast on QWER and regular cast on Shift+Q, Shift+W, Shift+E, Shift+R (ironically, the default smartcast keys)?
The Shift+key is a little bit harder to press and takes a little bit more time, sure. But you will only be using it in situations when you have time for it (if you don't, then you can't take your time to aim, then you're better off with simple smartcast).
The only problem I see with this approach is that you somehow cannot assign regular casting to Shift+key in-game - you need to change input.ini manually.
Doesn't it solve the problem?
P.s. I've been using this config for over half a year now, and haven't had any problems with it.