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I'm trying to build a fleet with heavily armoured and shielded transports. The idea is that the other ships in the fleet (mainly APB ships) go on with pounding the defenses until they are destroyed while the ATT (Assault Troop Transports) go in an drop troops even before all defenses are destroyed.
I have succeeded in getting them to drop troops AFTER all defenses are gone but not while the attacks are still going on.

In my fleet, i have planet buster ships which are loaded with planetary napalm so these need to get close to the planet in order to be effective while the APB ships need to keep more distance.

When i test and only use fleet of ATT's, they indeed attack and drop troops. A fleet of ATT's, APB's and Busters, keeps the ATT's back until the defenses are gone. I've tried to let them break formation but this doesn't achive anything:

Fleet strategy is capture planet:
Primary Movement Strategy is: Drop troops
Secondary Movement Strategy is: Point Blank

APB's ship strategy: Optimal Fire Range
Buster's ship strategy: Optimal Fire Range
ATT strategy: Capture Planet

Any idea as to what i need to change to have the ATT join in the attack and drop troops even before the defenses are destroyed?