All interesting points -- and really, the problem of the AI attacking force fields comes up whether the force fields are allowed to cover wormholes or not, so I'm not seriously considering turning off force fields that get too close to the wormholes any time soon.
My strategy for the above would be turrets and fighters. Possibly also a few starships if I had the resources. 2000 raiders is an awfully big wave for very early in the game, I tend to play on difficulty 7 or so and so don't tend to run into that. I could play on a higher skill level, but how I basically balance the game is so that I will win most of the time on 7, with it being an interesting challenge but not crazy difficult for me. That's my guideline for game difficulty in general, and the other difficulty levels are progressing up/down from that point. Sounds like you might also be playing against one of the Raider AI types, which have so much stronger waves, but weak defenses; definitely a challenge, and I suppose that would make grav turrets important all that much faster, as you say, though it's going to be a pretty tough challenge right at the very start.