Here is another opinion I have: if you are playing Synod, you should probably be playing some number of Thoughtcast. Synod is a greedy card and you should be rewarded for your efforts.
It doesn't hurt that Baral discounts Thoughtcast but does not discount Preordain.
I don't think that running 0 Preordain is ever correct, but you could run a split perhaps.
Edit - I'm not sold on Shoal like the OP is, but at least Thoughtcast recovers the lost CA while Preordain doesn't. But you still often need Preordain for t1 to get things going.