Having played an 80 world map with AI's at 7.3, where I took every planet, I have gained a wee bit of experience with heavily defended worlds.
The AI Progress finished around 4000+ or so, with the last 10 or so planets having around 5k of mark IV/V vessels. There are a couple of methods I have found useful.
Force field - 2 methods of use. First send thru a WH to activate the vessels on the other side, generally resulting in an attack by the AI that lessens its defenders of that wormhole. Second: A mark II or III FF thru followed by the attacking force. Secure the WH and build your beachhead.
Transport - 2 Methods of use. First: The ai will chase these with anything that comes into target acquisition range. Fly about 10 into the system, 5 go in one direction, 5 in the other, making sure to skim WH, and command center. Once done send the transport as far away as you can. Second: You can put all attackers less StarShips, Experimentals, and resistance fighters in transports. Thru the WH, move to a location mostly out of range, setup a defensive or move on top of the target you want to eliminate. I recommend disband the transport to unload, it is much quicker then hitting the unload button. In addition, the transports are rarely in good enough shape to re-use anyways.
Emp/Lightning Warhead - 1x Lightning Warhead into each guard post, 2x into the Spec Ops. If there is a Warhead interceptor, send an emp Warhead thru, followed by the lightning. If your defense is strong enough hit the command center (1x lightning is sufficient) the planet will empty if you have killed the guard posts, if not only the majority will attack you leaving some clean up.
Spider Turrets - Since these have effectively infinite range, kill engines and are not affected by force fields these are amazing for beachheads. A dead ship does not count against any limits and effectively response quickly. A ship with no engines, particularly one pulled out of range, stills counts against the total, but cannot harm you.
Cloaked raids - Take about 100 or so cloaked vessels thru the wh on low power, locate the guard posts, hit one at a time.
Atrieus