Needn't be excessively smart pathfinding, just don't ever fly through damaging terrain unless it's on manual.
Never? What about when your destination is surrounded by, or inside damaging terrain?
Or if the fleet itself is already inside damaging terrain?
Or the alternative route is much longer, or impossibly small to navigate at speed?
What about multiple overlapping terrains?
How do we account for terrain that disappears/appears/moves?
The rule bounds for SS's terrain and movement mechanics are so lax, that even a basic avoidance algorithm would be complex and computationally expensive.
I believe this is intentional; reliably avoiding hazards can be challenging even for the player, making it somewhat engaging gameplay. (Especially when combined with enemy fleet avoidance)