I don't think the directions they turn are random. They're evasion courses, controlled by the doctrine and fed data about the incoming weapons.
There is unfortunately no easy solution. To the greatest extent possible, you should spread out the ships. The further apart they start, the less of a chance for a collision. Apart from that, you may need to experiment with different formation shapes. Try running your scenario a few times, attacking from directions where attacks in-game are likely to come from and see which ships pile up, and how. Use that to change their position a little bit and see how it changes things.
In my experience, the worst position for ships is side by side. If you have to have them in close, stagger them laterally to give them all turning room. Another common problem is when the ships rejoin formation. If the ships are in a front-to back line, one ship may try to move through another to get in position. You can solve this by a linear offset, or by making sure the ships in the lead of the formation are also the fastest (and will arrive back on-station the quickest).
I've had far more success in ASW-Screen formations than I have with any sort of tight merchant convoy.
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