There's going to be a non-trivial rework of AI behaviour in the next patch, so we'll have to figure out new strategies for how to conquer planets. What I do right now (which works pretty well) is something like this.
Build a full cap of fleetships and a bunch of starships (I like carriers, shield and sniper most). Take your fleetball to the weakest adjoining planet and just sit there next to the wormhole. All the AI ships will come and attack my forces. If I am winning, then once the AI ships are destroyed, then take out the turrets. If I am losing, run away through the wormhole and rebuild. Rinse and Repeat.