Just a random thought.
What if you could move and cast at the same time after the 1 second mark ( or perhaps 0.5 sec )?
So, after the casting bar gets past 1 second. You can move without the casting getting interrupted. I think that this could deal with some of the headaches.
I am not sure at where the line is drawn currently, if you can move at like 0.10 or where it is. But I feel like even 0.5 could work.

It would just make things more fun overall imo too. Being a turret or immobile doesn't mean that you have to be taped to the floor necessarily.
I like to stand still and just unleash hell, but I also like to have some aspect of mobility going on.

I just think that this could be cool for casters overall. Just a silly thought ^^.