[Suicide Junkie]

Have you set the fighter strategies appropriately to match the ships?

Also, if you send the fleets in together, they should shoot the planet until all of the armed platforms are dead, then stop and wait for the troop transports.

You could include a small, slow, well armored troop transport in your assault fleet, too. It would force the rest of the fleet to back off once the platforms are gone, and you could follow up with the main occupying force later.
Presuming you are playing a mod in which ground combat is not always over instantly like stock has...