Mine the mid exit hexes with a B-17 or Catalina.
On my mine first play I used a B-17 to harass the invasion fleet. Later I thought to drop mines in the path of enemy carriers. but then I triggered the fleet retreat.
My bomber where all on their way back to refuel, and I panicked. I started to drop mines on the exit hexes in hope to slow down their retreat.
Quickly I noticed the enemy fleet is only using the mid exit hexes, ignoring all other hexes. Since those are only 5 hexes I realized the potential this huge critical tactical failure has for exploitation. A mistake like that deserves to be punished. So, I restarted and started right away mining all 5 mid exit hexes with a Catalina this time.
Worked like a charm. Destroyed every single ship. Because I crossed the mid of the map with surface vessels before triggering the retreat I provoked the Yamato task force to spawn.
Well, she went down as well

, but was a very hard kill indeed. Only torpedo bombers and my 2 subs could hurt her. There is no bonus for sinking her, so it was in all a very expensive victory in cost of mines and planes lost, but incredibility fun and sinking the Yamato is very cool.
The devs didn't like my strategy. I know it's cheesy, but I'm only used the tools the game gave me work with. Don't give me a hammer if you don't want me to break things.
About the triggering the fleet retreat. Seems it happened when the total number of enemy planes drops below a certain threshold. I specifically tried to avoid destroying any enemy flight groups, but instead just shoot them down to 1-4 planes each. Still caused the retreat.