not just move them all towards where they know the enemy is going to come from.
Why not, though? Would make sense by any warfare - to move your defences towards the advancing enemy. It's called intel and is used extensively in any war.
Besides, I wouldn't expect structures to move as fast as even the slowest ship, or even by themselves (say, towed by the builders). And certainly not as fast as an approaching phase jump towards that planet. It should be a
strategic relocation, not a tactical one.