Just some general notes:
1) Generally, Dyson sphere doesn't need defenses. The power could probably be best spent elsewhere.
2) Never put turrets or fortresses under human FFs. The drop in DPS kills any surviability payoff you might be looking for. Do, however, put engies and rebuilders under a FF near the turrets. Turrets can be rebuilt during a furball, and these little guys will happily try to keep rebuilding as many as possible.
3) Spire cities CAN be placed under human FFs (I do it all the time), but you'll probably need to micro the exo-waves just a bit. I usually have to manually target siege ships to clear them out, then let the autotargetting sort out most of the rest. You'll also need to vary the weapon loadout. I can offer some additional pointers if you're interested.
4) If the AI is swarming you with carriers, build an extra fortress or two on mission critical planets that aren't behind a chokepoint (higher the mark you can afford in metal and research, the better)
5) Don't go crazy trying to craft chokepoints on the galaxy map.
In regards to #5 - your preferred map style gives you fairly natural chokepoints, but you also have to take a lot of territory to establish them. You should generally be done taking planets about half way through Mk II AIP, so if you're only building up your empire to that point, it will be hard to progress further. Not impossible, but definitely more difficult. You may want to consider trying a more archipeligo strategy in the pocketed areas and make your primary defense be your turrets.
Speaking of turrets, if you use those as your primary defense, try to keep the turrets upgraded to AIP Tech Level + 1 (so level 2 turrets at game start, level 3 when AIP crosses to tech level 2, etc). This is because, in addition to doing less damage in general, lower mark units get a penalty to damage when firing at higher mark units, and the higher mark units get a damage bonus when firing back at the lower mark units. Keeping turrets upgraded based on AI tech level helps to mitigate this general problem.
Note that on chokepoints, I'd still build the obsolete turrets if you have the spare power. The auto targeting tries to maximize damage, and the damage bonus against low marks is very juicy to it. This makes them an excellent meat shield to soak up a couple stray shots with. On non-chokepoints, I'd only build them if it was a highly critical system for you (like an advanced factory system).