I have been experimenting with invasion/raid auto resolve and sitting in system doing nothing and it seems that you are better off not investing in any defense structures whatsoever other then a star fortress when you personally have to deal with a invasion by sitting on your colony since the scaling is so great even in a best case scenario the auto resolve with a size 10 colony with every defense structure fully upgraded coupled with alphas on the planetary shield plus ground batteries will still fold like a house of cards or be utterly crippled even if the intel screen says your ground defense is superior(perhaps that's a result of all opposing fleets attempting to invade individually).
If you sit in system you first notice how the invading fleets even if overwhelming are very reluctant to invade and spend their time chasing every last frigate down (I'm thinking hovering over your planet with your transponder off might also be a cause, flying a bit a way while still being within system seemed to solve the issue if there was no distraction npc allies), also it appears there is a bug where the event can conclude with the invasion succeeding and all invasions/raids getting cancelled if applicable(this has happened when I had overwhelming ground advantage and just sat in system watching) but you still retain control of the colony. finally with just passively watching in system even if you are massively superior ground defense wise the colony will quickly get disrupted in the majority of its industries and this can easily result in it folding if a single defense structure goes down(I don't include the ground defense from a space station since that will always fall before any invasion starts).
Almost forgot, Raids seem busted currently, even if you are at risk from them on the intel screen it seems they always resolve themselves after a while with no damage done to your colony aside from losing the star fortress when I imagine they should hit you with stab damage(I have gotten the raid successful message and checked my colony in question and there was no stab hit), currently I completely ignore them.